MAP4K1 (Mitogen-activated protein kinase kinase kinase kinase 1): MAP4K1 is a signaling protein, also known as HPK1 (Hematopoietic progenitor kinase 1), and a member of the STE20 kinase family. It plays a key role in immune cell signaling by regulating T-cell activation and differentiation through its upstream action in the MAP kinase pathway. Due to its function in modulating the immune response, MAP4K1 is considered a potential target for immunotherapy, especially in autoimmune diseases and cancer.