PDCD1 and cancer: CLL is a type of cancer where the bone marrow produces too many lymphocytes (a type of white blood cell), often leading to dysfunction of the immune system, which the literature suggests may be related to checkpoint molecules such as PD-1 (programmed cell death 1) and its ligand PD-L1, or CTLA-4 (T cell cytotoxic antigen 4), which may play a significant role in the progression of CLL.