Starting in 1979, the physicist Giorgio Parisi wrote a series of ground breaking papers introducing the idea of replica symmetry breaking, which allowed him to predict a solution for the Sherrington-Kirkpatrick (SK) model by breaking the symmetry of replicas infinitely many times. This is known as full-step replica symmetry breaking (FRSB). In this talk, we will provide a mathematical proof of Parisi's FRSB prediction at zero temperature for the more general mixed p-spin model. More precisely, we will show that the functional order parameter of this model is not a step function. This talk is based on joint work with Antonio Auffinger and Wei-Kuo Chen.