Fast-twitch myofibrils grow in proportion to Mylpf dosage in the zebrafish embryo.

Muscle cells become stronger by expanding myofibrils, the chains of sarcomeres that produce contraction. Here we investigate how Mylpf (Myosin Light Chain Phosphorylatable Fast) abundance impacts myofibril assembly in fast-twitch muscle. The two zebrafish Mylpf genes ( and ) are exclusively expressed in fast-twitch muscle.