Human fetal muscle and cultured myotubes derived from it contain a fetal-specific myosin light chain.

Human fetal muscles at ages 110, 125, and 132 days contain a fetal-specific myosin light chain. This light chain is absent in adult human muscle, copurifies with myosin, and is identified as a slow light chain because it reacts with purified antibody to chicken slow muscle light chains and does not react strongly with antibody to fast myosin light chains. This light chain is synthesized in cultures of fetal muscle along with normal myosin light chains. The presence of a fetal light chain in culture provides a marker for studies of human muscle disease in which it is important to know when or if the muscle makes a transition from embryonic or fetal expression to true adult phenotype.