A Ukrainian gas company with ties to former Vice President Joe Biden's son has repeatedly been mentioned in relation to President Donald Trump's impeachment.During a July 25 phone call, Trump urged Ukraine's president to launch investigations into former Vice President Joe Biden and his son, Hunter Biden, as well as a debunked conspiracy theory related to 2016.Trump wanted Ukraine to launch an investigation into Hunter's work for Burisma Holdings, a Ukrainian natural gas company.Hunter began working for Burisma in 2014. This was around the same time the former vice president was spearheading the Obama administration's efforts to pressure Ukraine to root out corruption.Trump and his personal lawyer Rudy Giuliani have suggested, without evidence, that Biden improperly pressured Ukraine to fire a prosecutor who had at one point been investigating Burisma Holdings.Though some ethics watchdogs have criticized Hunter's decision to work for Burisma in light of who his father is, there's no evidence of wrongdoing or illegal activity on his part or the former vice president's.And there's nothing concrete to support the suggestion Biden pressured Ukraine to take actions to the benefit of his son.With the Senate impeachment trial underway, Trump's defense team has continued to suggest the Bidens were guilty of corruption in Ukraine despite a lack of evidence.Visit Business Insider's homepage for more stories.A Ukrainian gas company called Burisma Holdings has repeatedly come up in relation to President Donald Trump's impeachment.In a July 25 phone call, Trump urged Ukrainian President Volodymyr Zelensky to launch investigations into former Vice President Joe Biden and his son, Hunter Biden, as well as a bogus conspiracy theory that Ukraine, not Russia, interfered in the 2016 election.Trump wanted Zelensky launch an inquiry into the Bidens in relation to Hunter's work for Burisma, despite no evidence of wrongdoing or illegal activity on the part of either of them.Here's what we know about the company and how it's involved in the back-and-forth between Trump and the Bidens.Fast facts about Burisma Holdings:Burisma Holdings is among Ukraine's largest independent natural gas companies.The company was founded in 2002 by Mykola Zlochevsky, an ally of the former Ukrainian President Viktor Yanukovychthe pro-Russia leader who was ousted in 2014 and has lived in exile in Russia ever since.Burisma is owned by the Cyprus-based offshore company Brociti Investments Limited, which records show is owned Zlochevsky, BuzzFeed News reported.Zlochevsky served as Ukraine's ecology minister under Yanukovych, assuming the role in 2010.Zlochevsky also fled the country not long after Yanukovych went into exile, according to The New York Times, as the office of Ukraine's prosecutor general opened multiple investigations into him and his businessesincluding suspicion of tax evasion and money laundering.What we know about Hunter Biden's role at Burisma:In April 2014, Biden's son Hunter joined the board of Burisma Holdings. Hunter served on the board until early 2019.At the time, a news release from the company said Hunter would be "in charge of the Holdings' legal unit and will provide support for the Company among international organizations."Hunter told the New York Times that the news release was not accurate and he was never in charge of the company's legal affairs.He joined the company about a month after Russia annexed Crimea, a cataclysmic moment that continues to put the US at odds with Russia and is linked to ongoing conflict in eastern Ukraine.During his time with Burisma, Hunter reportedly received compensation up to $50,000 a month.From the start, Hunter's role at Burisma was criticized by ethics watchdogs as a conflict of interest for his father, who was still vice president at the time and heavily focused on pressuring Ukraine to do a better job rooting out corruption. But some ethics watchdogs at the time also said that unless there was clear evidence Hunter got the job to influence US foreign policy then there was no cause for concern.His hiring by Burisma was seen as an attempt by the company to bolster its image and the perception it had strong ties to the US as the world vilified Russia for its annexation of Crimea, the Times reported.Yoshiko M. Herrera, a professor of political science at the University of Wisconsin at Madison and an an expert on Russia and Eurasia, told The Washington Post: "I think there is a conflict of interest even if it doesn't break any laws. It's a big deal. It's the vice president, who is the point person of the Obama administration's policy on Ukraine, and his son is suddenly hired to be a director on the board of Ukraine's largest private gas producer."With that said, Hunter has never been accused of wrongdoing regarding his work with Burisma.Hunter also saidhe only had one brief conversation with his father about Burisma which did not go into substantive details about the deal. Joe Biden has said he learned about his son's role at the company from news reports.Here's why this is now linked to Trump's impeachment:Trump and his attorney Rudy Giuliani have suggested that Biden improperly leveraged his role as vice president to push for the ousting of a man named Viktor Shokin as Ukraine's top prosecutor in order to help his son avoid getting caught up in corruption investigations.Trump has admitted that in a July 25 call with Ukrainian President Volodymyr Zelensky, who was elected in April, that he addressed investigating Biden and his son.The White House released a memo on the call that showed Trump repeatedly pressuring Zelensky to investigate the Bidens."There's a lot of talk about Biden's son, that Biden stopped the prosecution and a lot of people want to find out about that," Trump said to Zelensky on the call, according to the memo. "If you can look into itit sounds horrible to me."The call is also central to a whistleblower complaint from an intelligence official that says Trump, among other things, asked Zelensky to "initiate or continue an investigation into the activities of former Vice President Joseph Biden and his son, Hunter Biden."The complaint alleges Trump has used "the power of his office to solicit interference from a foreign country in the 2020 US election."The scandal surrounding the call and whistleblower complaint sparked an impeachment inquiry into Trump that ultimately led to his impeachment.Trump has tried to flip the situation around, contending that the real issue is the role Biden played in Ukraine as vice president and keeps pointing to Shokin's firing and Hunter's work for Burisma.After Shokin was appointed as Ukraine's prosecutor general in February 2015, he inherited the investigations into Zlochevsky. He also ultimately launched another probe into the profitable gas licenses that were awarded to Zlochevsky's companies as he served as a top minister in Yanukovych's government.But by March 2016, Shokin was ousted. Hundreds of Ukrainians had demonstrated in front of the president's office calling for Shokin to be booted and the Ukrainian parliament voted to accept his resignation.For months before that, the US and other countries had pressured for Shokin to be ousted because he didn't make a concerted effort to fight corruption. Biden, who was spearheading the Obama administration's Ukraine work, was at the center of these efforts, and threatened to withhold $1 billion in loan guarantees from Ukraine if Shokin wasn't fired.So, it's true that Biden was among those who pushed for Shokin to be fired as Ukraine's top prosecutor, but by the time this happened the probe into Burisma was dormant, according to Bloomberg.According to the Times, Ukrainian and American officials have also debated whether Shokin was using the threat of prosecution against Burisma in order to solicit a bribe.Daria Kaleniuk, co-founder of the Ukrainian Anti-Corruption Action Center told The Washington Post, "Shokin was not investigating. He didn't want to investigate Burisma. Shokin was fired not because he wanted to do that investigation, but quite to the contrary, because he failed that investigation."Yuriy Lutsenko, Ukraine's former prosecutor general who left the post at the end of August, told Bloomberg in an interview in May that neither Biden nor Hunter are the subject of investigations: "I do not want Ukraine to again be the subject of US presidential elections. Hunter Biden did not violate any Ukrainian lawsat least as of now, we do not see any wrongdoing. A company can pay however much it wants to its board."Lutsenko added: "At the end of the day, Shokin submitted his own resignation."Additionally, Lutsenko on September 26 told The Washington Post: "From the perspective of Ukrainian legislation, [Hunter Biden] did not violate anything."Lutsenko in an interview with the Los Angeles Times on September 29 echoed his previous comments and said that he'd not seen any evidence of wrongdoing on the part of either Biden.Trump on October 3 stood on the White House lawn and once again called on Ukraine to investigate the Bidens.On October 4, it was reported Ukraine's new prosecutor general, Ruslan Ryaboshapka, is reviewing past investigations into the owner of Burisma. This raised the possibility of inquiries being restarted, the Wall Street Journal reported.Ryaboshapka on October 4 also told Reuters he's not aware of any evidence of wrongdoing on Hunter's part and that he'd not been in touch with any foreign lawyers regarding the case.Multiple witnesses in the impeachment inquiry have said there's no evidence of illegal activity on the part of the Bidens in relation to Burisma.Witnesses have also tied Trump's decision to freeze roughly $400 million in military aid to Ukraine to his call for investigations, suggesting there was an explicit quid pro quo. In short, they've alleged Trump withheld the aid as part of an effort to pressure Ukraine into launching an investigation that would smear Biden's name and benefit the president politically.In the Senate impeachment trial, Trump's defense team has continued to shift attention away from the president and toward Hunter and the former vice president. Biden has rejected suggestions from Republicans that he should testify in the impeachment trial. "I have nothing to defend. This is all a game," Biden told reporters on January 27.SEE ALSO:'Take her out': New recording appears to feature an angry Trump telling associates to 'get rid of' the US's ambassador to Ukraine after he was told she bad-mouthed himJoin the conversation about this storyNOW WATCH: Extremists turned a frog meme into a hate symbol, but Hong Kong protesters revived it as an emblem of hope Click here to read full news..