Laura Roslin
From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
Laura Roslin is in her mid-40s with some 20 years of political experience behind her. Effectively the the 43rd in-line for the Presidency of the 12 Colonies should anything happen to President Adar, she was serving at Education Minister at the time of the renewed Cylon attack on the colonies.
Immediately prior to the attack, Roslin had been diagnosed with breast cancer (Mini-Series), which had advanced far enough as to be inoperable. She apparently contracted it some time following her last test, some five years before (Act of Contrition)