RB1 and neuroendocrine carcinoma: The IHC distinction of NEC from G3 NET largely relies on protein correlates of these molecular genetic events, with mutant-pattern p53 staining (either missense mutation- or null-pattern) and/or Rb loss being supportive of but not diagnostic for the diagnosis of NEC (Basturk et al. 2015, Konukiewitz et al. 2017, Bellizzi 2020b).