SALL1 and Townes-Brocks syndrome: In fore-limbs of Sall1 and Sall3 double knock-out mouse embryos, the anterior digits and most of the carpal bones are lost [29] while Townes-Brocks syndrome, in which truncated forms of Sall1 are produced [30] is characterised by thumb malformations, mostly triphalangeal thumbs, which might be considered to represent a change in digit identity.