The striated muscle sarcomeres are highly organized structures composed of actin (thin) and myosin (thick) filaments that slide past each other during contraction. The integrity of sarcomeres is controlled by a set of structural proteins, among which are titin, a giant molecule that contains several immunoglobulin (Ig)-like domains and associates with thin and thick filaments, and [alpha]-actinin, an actin cross-linking protein. Mutations in several sarcomeric and sarcolemmal proteins have been shown to result in muscular dystrophy and cardiomyopathy. On the other hand, the disease genes underlying several disease forms remain to be identified. Here we describe a novel 57 kDa cytoskeletal protein, myotilin. Its N-terminal sequence is unique, but the C-terminal half contains two Ig-like domains homologous to titin. Myotilin is expressed in skeletal and cardiac muscle, it co-localizes with [alpha]-actinin in the sarcomeric I--bands and directly interacts with [alpha]-actinin. The human myotilin gene maps to chromosome 5q31 between markers AFM350yB1 and D5S500. The locus of a dominantly inherited limb-girdle muscular dystrophy (LGMD1A) resides in an overlapping narrow segment, and a new type of distal myopathy with vocal cord and pharyngeal weakness (VCPMD) has been mapped to the same locus. The muscle specificity and apparent role as a sarcomeric structural protein raise the possibility that defects in the myotilin gene may cause muscular dystrophy.