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Calponin is a family of actin filament-associated proteins which regulate smooth muscle cell contraction. Three isoforms of calponin exist: calponin h1 (CNN1) , calponin h2 (CNN2) and calponin 3 (CNN3). Calponin 1 and calponin 2 are predominately expressed in smooth muscle cells and cardiac muscle cells, respectively. Calponin 3 is highly expressed in many tissues including articular cartilage and brain.