The embryological basis of limb abnormalities in Fanconi anemia patients is thought to be based on the complex interactions between the apical ectodermal ridge (where Fanconi anemia genes are expressed) and both the mesoderm (where Spalt-like 4 (SALL4) and Sonic hedgehog (SHH) are located and which are responsible for radial ray deficiency, thumb polydactyly, and triphalangism) and the dorsoventral axis (an error in that axis leads to dorsal dimelia).