TP53 and retinoblastoma: Mutations in cell cycle regulatory pathway genes are also found in GEP-NECs; specifically, recurrent mutations in TP53, which encodes p53, a transcription factor and regulator of apoptosis (Shaw et al., 1992), and in RB (retinoblastoma), which encodes a key tumor suppressor that regulates the cell cycle (Whyte et al., 1988).