The expression pattern of nodal and lefty in the mouse mutant Ft suggests a function in the establishment of handedness.

We have investigated the expression of left/right (L/R) asymmetry markers, nodal and lefty, in the situs inversus mouse mutant Fused toes (Ft). Both genes exhibited bilateral expression in the lateral plate mesoderm (LPM) at developmental stages whereas in wildtype embryos these genes were found to be expressed exclusively in the left LPM. Inspection of tail location and primitive heart tube looping, structures known to be handed in their orientation, documented a random orientation of these structures. Crossing of the Ft mutation into a different genetic background resulted in a strong reduction of this random orientation. Although the major fraction of these individuals still displayed nodal and lefty on both sides of the LPM, expression was almost always found to be weaker in the right LPM. These results suggest that the establishment of asymmetry is independent of nodal or lefty signals. However, handed asymmetry, which means consistent L/R differences, such as the dextral looping of the primitive heart tube or the right-oriented tail, is directed by differences in the L/R expression pattern of these two genes.