What is distinctive about Giovanni Pierluigi da Palestrina's Missa brevis?

Giovanni Pierluigi da Palestrina’s Missa brevis is notable for being a work in a "free style," meaning its musical material is entirely original rather than derived from a pre-existing cantus firmus or borrowed melodies, as was common in many of his other masses. This composition, written for four voices, is considered the best-known example of this approach within his extensive body of work.1