We identified titin as a chromosomal protein using a human autoimmune scleroderma serum. We cloned the corresponding gene in the fruitfly, Drosophila melanogaster. We have demonstrated that titin is not only expressed and localized in striated muscle but is also distributed uniformly on condensed mitotic chromosomes using multiple antibodies directed against different domains of both Drosophila and vertebrate titin. Titin is a giant sarcomeric protein responsible for the elasticity of striated muscle. Titin may also function as a molecular scaffold during myofibril assembly. We hypothesize that titin is a component of chromosomes that may function to determine chromosome structure and provide elasticity, playing a role similar to that proposed for titin in muscle. We have identified mutations in Drosophila Titin (D-Titin) and are characterizing phenotypes in muscle and chromosomes.