PSMB9 and Merkel cell skin cancer: At baseline, the MCC lines exhibited low expression of HLA-B, TAP1, TAP2, PSMB8, and PSMB9, compared with control epidermal keratinocytes and dermal fibroblasts (30, 31), which are candidates for the cell of origin of MCPyV– and MCPyV+ MCC (ref. 32 and Figure 2A).