The PD-1 gene (PDCD1) has been shown to be a haploinsufficient tumor suppressor that is frequently altered in T-cell lymphomas, and it is mutationally truncated, as well as heterozygously and biallelically deleted, in advanced MF and in SS patients (10–40% of cases) [26,27,32,38,43,145]. Here, PDCD1 is linked to T-cell non-Hodgkin lymphoma.