The UNC-45 or small muscle protein 1 of C.elegans is expressed in two forms from different genomic positions in mammals, as a general tissue protein UNC-45a and a specific form Unc-45b expressed only in striated and skeletal muscle. All members carry up to three amino-terminal tetratricopeptide repeat (TPR) domains towards their N-terminal, a UCS domain at the C-terminal that contains a number of Arm repeats pfam00514 and this central region of approximately 400 residues. Both the general form and the muscle form of UNC-45 function in myotube formation through cell fusion. Myofibril formation requires both GC and SM UNC-45, consistent with the fact that the cytoskeleton is necessary for the development and maintenance of organized myofibrils. The S. pombe Rng3p, is crucial for cell shape, normal actin cytoskeleton, and contractile ring assembly, and is essential for assembly of the myosin II-containing progenitors of the contractile ring. Widespread defects in the cytoskeleton are found in null mutants of all three fungal proteins. Mammalian Unc45 is found to act as a specific chaperone during the folding of myosin and the assembly of striated muscle by forming a stable complex with the general chaperone Hsp90. The exact function of this central region is not known.