Calponin 1 is a basic smooth muscle protein that in humans is encoded by the CNN1 gene. The expression of CNN1 is specific to differentiated mature smooth muscle cells, suggesting a role in contractile functions. Calponin 1 is up-regulated in smooth muscle tissues during postnatal development with a higher content in phasic smooth muscle of the digestive tract. Calponin binds actin to promote and sustain polymerization. The binding of calponin to F-actin inhibits the MgATPase activity of smooth muscle myosin. Calponin binds F-actin through two sites at residues 144-162 and 171–188 in chicken calponin 1. The two actin-binding sites are conserved in the three calponin isoforms.