Periostin was originally isolated as a osteoblast-specific factor that functions as a cell adhesion molecule for preosteoblasts and is thought to be involved in osteoblast recruitment, attachment and spreading. Additionally, periostin expression has previously been shown to be significantly increased by both transforming growth factor beta-1(TGFβ1) and bone morphogenetic protein (BMP)-2. Likewise the endocardial cushions that form within embryonic heart tube (embryonic day (E)10–13) are formed by the recruitment, attachment and spreading of endocardial cells into the overlying extracellular matrix, in response to secreted growth factors of the TGFβ and BMP families. In order to determine whether periostin is similarly involved in heart morphogenesis, in situ hybridization and reverse transcription–polymerase chain reaction were used to detect periostin mRNA expression in the developing mouse heart. We show for the first time that periostin mRNA is expressed in the developing mouse embryonic and fetal heart, and that it is localized to the endocardial cushions that ultimately divide the primitive heart tube into a four-chambered heart.