Target CEO, president and chairman Gregg Steinhafel has resigned, effective immediately, the retailer announced on Monday, just five months after Target suffered one of the worst security breaches in U.S. history. The breach resulted in the theft of payment or personal data from between 40 million and 110 million customers. “The last several months have tested Target in unprecedented ways,” Steinhafel wrote in a letter to the company’s board of directors. “From the beginning, I have been committed to ensuring Target emerges from the data breach a better company … With several key milestones behind us, now is the right time for new leadership at Target.” Steinhafel’s resignation comes two months after the company’s chief information officer Beth Jacob also resigned in the wake of the breach. Target CFO John Mulligan will serve as interim president and CEO until the board appoints a new CEO.