COL3A1 and Merkel cell skin cancer: The following criteria were used to define distinct MCC subpopulations: EpiMCC—non-zero expression of CCER2 and KRT5, along with the expression of either CALML3 or CALML5; cMCC—co-expression of CCER2 and KRT20, with no detectable expression of COL3A1, COL1A1, or KRT5; and vasMCC—non-zero expression of both CCER2 and KRT20, along with the expression of either COL3A1 or COL1A1.