In contrast to the MCC model, the engrafted fraction of CD8+ T cells in the MPC iPDX peripheral blood demonstrated a significant increase from early to late engraftment with a 1:75 ratio of CD8+ and CD4+ T cells at 2 weeks to 2:1 ratio at 8 weeks (S3D Fig, middle and right panel). This evidence concerns the gene CD4 and Merkel cell skin cancer.