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