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 signalling and in maintaining cellular calcium balance. STIM1 is essential 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.