Troponin I (TnI) is one of three subunits that form the troponin complex of the thin filaments of striated muscle. Skeletal muscle troponin I exists in two isoforms, TnI-fast and TnI-slow, expressed respectively in fast and slow-twitch muscle fibers. The two isoforms are characterized by molecular weights ~ 21 kDa (fast) and 22 kDa (slow). The ratios of slow and fast isoforms vary with muscle source and species. Skeletal muscle TnI offered by Life Diagnostics, Inc., is a mixture of slow and fast isoforms. It is purified by ion-exchange chromatography in conjunction with calcium dependent affinity chromatography on troponin-C agarose.