Abnormal nodal flow precedes situs inversus in iv and inv mice

We examined the nodal flow of well-characterized mouse mutants, inversus vi scerum (iv) and inversion of embryonic turning (inv), and found that their laterality defects are always accompanied by an abnormality in nodal flow. In a randomized laterality mutant, iv, the nodal cilia were immotile and th e nodal flow was absent. In a situs inversus mutant, inv, the nodal cilia w as motile but could only produce very weak leftward nodal flow. These resul ts consistently support our hypothesis that the nodal flow produces the gra dient of putative morphogen and triggers the first L-R determination event.