LCK (Lymphocyte-Specific Protein Tyrosine Kinase): LCK is an enzyme that plays a key role in the T-cell receptor (TCR) signaling pathway. It phosphorylates important proteins to initiate T-cell development and activation, essential for the adaptive immune response. Dysregulation of LCK can affect immune function and is linked to autoimmune diseases and immunodeficiency. Because of its central role, LCK is a target for immunotherapy in cancer and autoimmune disorders.