Titin, the largest protein identified to date (over 1 micron long, almost 3 
million daltons in mass) is the third most abundant component of the sarcomere. 
In the mature myofibril, titin molecules span from M line to Z line, forming a 
third filament system which provides sarcomeric alignment and elastic recoil. In 
the developing sarcomere, accumulating evidence from studies both in vivo and in 
vitro implicates titin as part of a morphogenetic scaffolding, upon which 
critical events in myofibrillogenesis are coordinated in a time- and 
space-dependent manner.