In the myofibril of striated muscle, there are three myofilaments thick, thin, and fine myofilaments. Titin (connectin) is a giant elastic contractile protein, with molecular weight of 3000 kD and length of 0.9pm, and forms fine myofilament extended from M-line to Z-line in the myofibril. It may play roles in maintaining thick myofilament in the middle of sarcomere, acting as molecular template for assembly of thick myofilament, and modulating the myosin activity.