Titin is the largest polypeptide yet described (relative molecular mass 
approximately 3 x 10(6); refs 1, 2) and an abundant protein of striated muscle. 
Its molecules are string-like and in vivo span from the M to Z-lines. I-band 
regions of titin are thought to make elastic connections between the thick 
filament and the Z-line, thereby forming a third type of sarcomere filament. 
These would centre the A-band in the sarcomere and provide structural continuity 
in relaxed myofibrils. The A-band region of titin seems to be bound to the thick 
filament, where it has been proposed to act as a 'molecular ruler' regulating 
filament length and assembly. Here, we show that partial titin complementary 
DNAs encode a regular pattern of two types of 100-residue motif, each of which 
probably folds into a separate domain type. Such motifs are present in several 
evolutionarily divergent muscle proteins, all of which are likely to interact 
with myosin. One or both of the domain types is therefore likely to bind to 
myosin.