Whether Ellen Pao was discriminated against in her Silicon Valley venture capital firm for being a woman is at the heart of the Pao-Kleiner trial, which began Tuesday in San Francisco. The trial comes a time when there's a gender gap in American tech companies, and when women are reportedly leaving in "droves."

Ellen Pao’s attorney argued in opening statements that while she was not much different from the partners of venture capital firm Kleiner Perkins Caufield & Byers, she was not selected to move up like her male peers were.

Pao originally filed a lawsuit in 2012, claiming that she had been discriminated against because she was a woman, and retaliated against after she ended an affair with a married male colleague, Ajit Nazre. When she told her bosses about the issue, they told her not to complain and gave her bad reviews, which led to her being fired.

Kleiner Perkins claims Pao “could not demonstrate the skills necessary for success as an investing venture capitalist,” according to Wired, and that she often had conflicts with male and female colleagues, and failed to gain their trust.

While Pao was given negative performance reviews for being “difficult to work with,” her male colleagues were often treated less harshly for being “blunt or overbearing” and “arrogant.” Pao’s attorney claims there is proof the firm systematically treated women unfairly, including several female partners who were never promoted to managing partners.

One of the trial’s first witnesses will be Trae Vassallo, according to Re/code, Pao’s former co-worker and a respected former partner who, the site says, led high-yielding investments in Nest and Dropcam, among others.

Vassallo left the company after she was not selected for being on a committee that selected new investments. She had also had unwanted contact with Nazre, and claimed that he once put his legs on her inappropriately at a business dinner.