STK4 (serine/threonine kinase 4), also known as MST1, is a critical regulator of multiple cellular processes including apoptosis, tissue homeostasis, and immune function. As a core component of the Hippo signaling pathway, STK4 forms complexes with regulatory proteins to phosphorylate and activate downstream effectors like LATS1/2, ultimately leading to YAP1 inactivation and growth suppression 1. The kinase promotes apoptosis through caspase-mediated cleavage, nuclear translocation, and phosphorylation of targets including histone H2B and FOXO transcription factors 2. STK4 regulates metabolic processes by controlling mitochondrial function in adipose tissue, where its inhibition increases mitochondrial mass and energy expenditure while conferring resistance to diet-induced obesity 3. The protein also modulates autophagy and cellular senescence through phosphorylation of RNA-binding proteins like AUF1, affecting glucose metabolism 2. Clinically, STK4 deficiency causes a severe primary immunodeficiency (Immunodeficiency 110) characterized by lymphoproliferation, autoimmunity, and increased susceptibility to EBV-associated malignancies 45. Additionally, STK4 contributes to endometrial receptivity through phosphorylation of Nur77, affecting embryo implantation success 6. Pharmacological inhibition of STK4 shows therapeutic potential for enhancing tissue repair and regeneration 1.