Bmp7/Tsg, Bmp7/Shh mutants and compound Bmp7 homozygous/Bmp4 heterozygous conditional inactivation in the region of the trunk to tail transition resulted in the fusion of the hindlimbs (sirenomelia) due to the absence of ventral lateral mesoderm22,23,43 and, thus, of the pericloacal mesoderm that builds the GT20, which is also absent from Tgfbr1 null mutant embryos21. The gene discussed is BMP7; the disease is sirenomelia.