Six "native" isomyosins of characteristic electrophoretic mobility are present in various proportions according to the muscle type (fast, slow or mixed). SM2 and SM1 contain MHC1; IM contains MHC2A; FM1, FM2 and FM3 contain MHC2A and MHC2B. SM2, SM1 and IM contain MLC1s; SM1, IM, FM2 and FM3 contain MLC1f; FM2 and FM1 contain MLC3f. The six isomyosins appear to separate on the base of their myosin light chain content.