STIM1 (Stromal Interaction Molecule 1): STIM1 is a protein that serves as a key regulator of calcium (Ca²⁺) entry into cells. It plays a vital role in calcium signaling and maintaining cellular calcium balance. STIM1 is crucial for processes such as muscle contraction, gene expression, and cell growth, and mutations in STIM1 are associated with immune system dysfunction and muscle-related disorders.