TNNI1 encodes slow skeletal muscle troponin I, the inhibitory subunit of the sarcomeric thin filament troponin complex that regulates calcium-dependent muscle contraction 1. As the slow-twitch isoform, TNNI1 is predominantly expressed in slow-twitch skeletal myofibers and confers calcium-sensitivity to striated muscle actomyosin ATPase activity 1. TNNI1 mutations cause distinct skeletal muscle diseases: recessive loss-of-function variants (p.R14H, p.R14C) manifest as early-onset progressive weakness with reduced calcium responsiveness and rod formation, potentially reversible by slow skeletal muscle troponin activators; dominant gain-of-function variants (p.R174Q, p.K176del) cause muscle cramping and myalgias with increased calcium sensitivity, responsive to mavacamten 2. These pathomechanisms reflect TNNI1's critical role in thin filament calcium regulation. Beyond muscle function, TNNI1 shows aberrant expression in various human tumors, where it promotes cell proliferation and potentiates oncogenic mutations, suggesting an unexpected nuclear-localized proliferative function distinct from its canonical muscle regulatory role 3. Localization to chromosome 1 4 and tissue-specific expression patterns 5 characterize this medically significant gene affecting both skeletal muscle disease and potentially cancer pathogenesis.