Nodal signaling and axis formation in the mouse

Members of the transforming growth factor-J3 (TGF-[3) superfamily of secreted growth factors function as key signaling molecules guiding morphogenetic processes throughout development (for review, see Hogan 1997). Work from our lab has focused on the nodal gene, origi-nally identified ue to its close linkage with the 413.d proviral integration site (Zhou et al. 1993; Conlon et al. 1994). Homozygous 413. d mutant embryos lacking nodal expression fail to initiate primitive streak formation and are arrested at the gastrulation stage of development (Conlon et al. 1994). The mouse nodal gene represents he prototype of a discrete subfamily of TGF-[3 proteins. To date, nodal ho-mologs have been cloned from chick, Xenopus, and ze