B2M and pachyonychia congenita: In particular, B2M loss has been described as a mechanism of resistance to programmed death-1 (PD-1) inhibition in melanoma and HLA class I antigen–processing machinery alterations more broadly have been associated with early disease recurrence in PC,18-20 raising the possibility that this defect resulted in loss of immune surveillance, even without the selective pressure of immune checkpoint inhibition.