The retinoblastoma protein, RB, is required for gastrointestinal endocrine cells to exit the cell cycle, but not for hormone expression.
Wang Y, Ray SK, Hinds PW, Leiter AB. The retinoblastoma protein, RB, is required for gastrointestinal endocrine cells to exit the cell cycle, but not for hormone expression. Dev Biol. 2007 Nov 15; 311(2):478-86.