RB1 and Merkel cell skin cancer: MCPyV+ MCC is a low–tumor mutational burden (low-TMB) subtype driven by 2 viral antigens: large T antigen (LT), which inactivates RB1 (14), and small T antigen (ST), which has numerous functions, including recruitment of MYCL, a MYC paralog, to chromatin-modifying complexes (15).