sentence1: The Retinoblastoma protein, pRb, was among the first recognized tumor suppressor proteins, and loss or repression of pRb function is thought to play a causative role in most human cancers.
sentence2: Mutation of the retinoblastoma tumor susceptibility gene (RB1) is the rate-limiting step in the genesis of retinoblastoma and over 90% of human tumors exhibit reduced pRB function.