Smooth muscle α actin is specifically required for the maintenance of lactation

► Smooth muscle α-actin (Acta2) −/− dams exhibit a nursing phenotype. ► Pups nursed by Acta2−/− dams are unable to thrive postpartum. ► Acta2−/− mammary glands exhibit precocious involution. ► Developmental reduction of myoepithelial cells in Acta2−/− mammary glands. ► Acta2 mediates myoepithelial contraction in an isoform specific manner.