The most primitive skeletal pattern consists of ribs projecting from vertebrae, from the head through to the tail. In mice, ribs are restricted to the thoracic region, and the axial skeleton consists of seven cervical, 13 thoracic, six lumbar, four sacral, and a variable number of caudal vertebrae. Hox genes control the patterning of the skeleton, but exactly how they achieve this has been confused by the redundant function of genes within the four chromosomal linkage groups, A–D, which arose during evolution by genomic duplication events. In the July 18
Wellick...