['Smad4 is a central mediator of transforming growth factor-β/bone morphogenetic protein signaling that controls numerous developmental processes as well as homeostasis in the adult. The present studies sought to understand the function of Smad4 expressed in vascular smooth muscle cells (VSMC) in vascular development and the underlying mechanisms.', 'Breeding of Smad4(flox/flox) mice with SM22α-Cre mice resulted in no viable offspring with SM22α-Cre;Smad4(flox/flox) genotype in a total of 165 newborns. Subsequent characterization of 301 embryos between embryonic day (E) 9.5 and E14.5 demonstrated that mice with SM22α-Cre;Smad4(flox/flox) genotype died between E12.5 and E14.5 because of decreased cell proliferation and increased apoptosis in the embryonic heart and arteries. Additionally, deletion of Smad4 more specifically in SMC with the inducible smooth muscle myosin heavy chain (SMMHC)-Cre mice, in which decreased cell proliferation was observed only in the artery but not the heart, also caused lethality of the knockout embryos at E12.5 and E14.5. The Smad4-deficient VSMC lacked smooth muscle α-actin filaments, decreased expression of SMC-specific gene markers, and markedly reduced cell proliferation, migration, and attachment. Using specific pharmacological inhibitors and small interfering RNAs, we demonstrated that inhibition of transforming growth factor-β signaling and its regulatory Smad 2/3 decreased VSMC proliferation, migration, and expression of SMC-specific gene markers, whereas inhibition of bone morphogenetic protein signaling only affected VSMC migration.'] Is vascular smooth muscle cell Smad4 gene important for mouse vascular development?