Digit evolution and HoxD expression. (a) In the evolution of the forelimb in different monkey species, morphological variation in digits 2-5 and the distal forearm (dark red) is correlated, whereas variation in digit 1 (green) corresponds to other independent regions. This phylogenetic pattern can be explained by variations in the late expression of HoxD-11 in the distal forearm and digits 2-5. (b) Domains of late HoxD-11 expression in an embryonic mouse paw. HoxD-11 expression is indicated by brown staining. Digits are numbered in the same order as in (a). The position where the thumb will develop is on the left. Modified from .