PDCD1 and melanoma: Some other factors indicating cytotoxic T cell activity, such as the level of granzymes, perforins, and IFN-γ, remain in the theoretical phase,403 and genetic PD-1 resistance-predicting markers such as β-2-microglobulin (B2M), JAK1, and JAK2, which have been validated in other tumors, such as melanoma, have not been confirmed to have similar effects in CRC, although the absence of B2M might correlate with a better clinical outcome.425,426