STAC (SH3 and Cysteine-Rich Domain): STAC is a protein — specifically STAC3 in humans — that plays a key role in skeletal muscle contraction. It is essential for excitation–contraction coupling, the process that links electrical signals to muscle movement. Mutations in the STAC3 gene can lead to Native American myopathy, a rare congenital condition marked by muscle weakness, skeletal abnormalities, and increased risk of malignant hyperthermia. Studying STAC3 is vital for understanding muscle physiology and related disorders.