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 an important role in immune cell signaling, regulating T cell activation and differentiation by acting upstream in the MAP kinase pathway. Through its role in modulating immune responses, MAP4K1 is considered a potential target for immunotherapy, particularly in autoimmune disorders and cancer.