CD4 and Merkel cell skin cancer: Although the correlation between CD4 counts and MCPyV copies was not shown in the present study (Figure 2) and the detailed immune status was not determined in the HIV-1-negative patients, the significantly different positivity of MCPyV DNA in the serum between HIV-1-positive and negative patients suggests that an individual’s immune status is associated with the production of MCPyV which will be a crucial factor in elucidating MCC pathogenesis.