Uber CEO Travis Kalanick is stepping aside for an indefinite period as the ride-hailing giant fights off toxic work environment claims. The ultimate responsibility, for where we’ve gotten and how we’ve gotten here rests on my shoulders,” Mr. Kalanick wrote in the email. “There is of course much to be proud of but there is much to improve. It all began in February when a former employee penned a blog post about sexual harassment she received during her tenure at the company. Over the last few months, Uber has fired 20 employees in the wake of an investigation by ex-Attorney General Eric H. Holder. Kalanick's return date is not specified, and his responsibilities will undergo "review" to possibly shift some duties to other executives.