SP1 and Merkel cell skin cancer: We confirmed that IL-33/ST2 activated both pathways in MCC cells; this may explain the stimulatory effect on the MCPyV promoters, which contains putative binding sites for NF-κB and for ERK1/2 MAPK-regulated transcription factors, such as AP1, ATF/CREB, c-MYC, ELK1, and SP1 [73,74].