Experimental manipulations affecting the length of the vertebrate tail. (A) Increasing RA exposure in mouse embryos leads to progressive loss of caudal and sacral vertebrae. s1 indicates first sacral vertebrae and c1 indicates first caudal vertebrae. Data adapted from Shum et al. 1999 . (B) Hoxb13 knockout (Hoxb13 KO) in the mouse increases caudal vertebrae number by 2 and causes more barrel-shaped as opposed to hourglass-shaped vertebrae. Bars indicate experimental marking of equivalent numbered vertebrae; arrowheads indicate caudal vertebra #30 in both wildtype (WT) and Hoxb13 KO; asterisks indicate two additional caudal vertebrae. Data adapted from Economides et al. 2003 . (C) Precocious ectopic overexpression of Hoxb13 in the mouse causes prematurely truncated tails. Data adapted from Young et al. 2009 . RA, retinoic acid.