STAC (SH3 and Cysteine-Rich Domain): STAC is a protein—specifically STAC3 in humans—that plays a crucial role in skeletal muscle contraction. It is essential for excitation–contraction coupling, the process that connects electrical signals to muscle movement. Mutations in the STAC3 gene can cause Native American myopathy, a rare congenital disorder characterized by muscle weakness, skeletal abnormalities, and an increased risk of malignant hyperthermia. Research on STAC3 is important for understanding muscle physiology and related diseases.