Goosecoid (gsc)is expressed in the organizer region of vertebrate embryos undergoing the movements of gastrulation. Likewise, the early heart tube (8.5-9.5 dpc) undergoes a similar process of looping to bring the atrial region cranial and dorsal to the ventricular region, eventually giving rise to the four chambered heart. In order to determine whether gsc is similarly involved in heart morphogenesis, in situ hybridization and RT-PCR were used to detect gsc expression in the embryonic mouse heart. We have shown, for the first time, that gsc mRNA is expressed in the developing mouse heart, and is localized to the sites that divide the primitive heart tube into a four chambered heart.