Former U.S. special envoy to Ukraine Kurt Volker testified in an impeachment hearing Tuesday that allegations against Joe Biden and former Ambassador to Ukraine Marie Yovanovitch, which were promoted by former Ukrainian Prosecutor General Yuriy Lutsenko and spread in the U.S. by Rudy Giuliani, are "self-serving and not credible."

Why it matters: These allegations are now at the heart of the impeachment inquiry, as President Trump and Rudy Giuliani have come under scrutiny for working to push Ukraine to investigate Biden for corruption. Volker, who is one of the witnesses called by Republicans, is one of multiple officials who have testified that the allegations against Biden and Yovanovitch are baseless.

Go deeper: Live updates from Volker and Tim Morrison's testimony