Carl Djerassi, who was called “The Father of the Pill,” died Friday at his home in San Francisco. He was 91.

Djerassi was 28 and working for a small chemical company in Mexico City in 1951 when he helped create norethindrone, a synthetic hormone that became a key ingredient in oral contraceptives popularly known as the Pill.

The cause of death was complications from liver and bone cancer, said his son, Dale.