Integrins are membrane receptors that mediate cell-cell or cell-matrix adhesion. All integrins are transmembrane heterodimers composed of α and β subunits that are connected to the cytoskeleton. In mammals, at least 17 α subunits and 8 β subunits have been identified, and these proteins can heterodimerize to form at least 22 different receptors. The β1 subgroup of the integrin receptors consists of at least 6 different dimer combinations. A variety of signal transduction proteins have been shown to bind the cytoplamic domain of β1 integrins. These include melusin, ILK, ICAP, and RACK1. Melusin is expressed preferentially in muscle and heart, and contains putative SH3 domain binding motifs in the N-terminal region, two putative SH2 binding sites, and a C-terminal acidic amino acid stretch (CAAS) similar to the calcium binding proteins, calreticulin and calsequestrin. In muscle, melusin protein is localized in two rows flanking α-actinin at Z-lines, and melusin mRNA is upregulated during neonatal development. Differentiation of C2C12 murine myogenic cell line by serum starvation also upregulates melusin protein and mRNA. Thus, melusin may be an important signal transducer for β1 integrins during muscle development.