'Following the rise of the Galactic Empire, the 501st transitioned into a stormtrooper unit, like other clone units. Given new armor and weapons, the 501st faithfully served the Empire, operating under the direct command of the Sith Lord Darth Vader. After the Kamino uprising, the 501st became the last remnant of the Grand Army of the Republic fully composed of Jango Fett clones'

Two can play the quote game. :P

By 0 BBY, Vader began to comb through the ranks of other legions to pull non-fett clones into the 501st. Before then, it was a pure Fett clone legion.