Calcium-dependent contraction of vertebrate striated muscle is regulated in part through the interaction of the troponin protein complex with tropomyosin and the actin-myosin myofibril. The troponin complex consists of three proteins, troponin I (TnI), troponin C (TnC), and troponin T (TnT). TnI is an actin-binding protein that enables the troponin complex to physically inhibit the interaction of myosin and actin, thus preventing muscle contraction in the absence of excitation-induced elevated intracellular Ca 2+. TnC is a calcium-binding protein that can, in the presence of elevated intracellular Ca 2+, reverse the TnI-induced inhibition of actin-myosin binding, thus initiating muscle contraction. TnT is a tropomyosin-binding protein that is thought to position the troponin complex on the myofibril. The troponin proteins are encoded by three distinct gene families. The two TnC genes encode distinct protein isoforms found either in fast skeletal muscle or slow skeletal and cardiac muscle. The TnI and TnT gene families each consist of three separate genes encoding protein isoforms found in cardiac, slow skeletal, or fast skeletal muscle, respectively. The murine TnC and TnI genes have all been cloned and, with the exception of fast skeletal muscle troponin C, chromosomally mapped, while none of the TnT genes have been mapped in the mouse genome. Fast skeletal muscle troponin T cDNAs encoding various protein isoforms have been