RBT (RABO-TORCIDO), A NEW MOUSE SKELETAL MUTATION INVOLVED IN ANTEROPOSTERIOR PATTERNING OF THE AXIAL SKELETON, MAPS CLOSE TO THE TS (TAIL-SHORT) LOCUS AND DISTAL TO THE SOX9 LOCUS ON CHROMOSOME-11

Rbt (Rabo torcido) is a new semidominant mouse mutant with a variety o f skeletal abnormalities. Heterozygous Rbt mutants display homeotic an teroposterior patterning problems along the axial skeleton that resemb le Polycomb group and trithorax gene mutations. In addition, the Rbt m utant displays strong similarities to the phenotype observed in Ts (Ta il-short), indicating also a homeotically transformed phenotype in the se mice. We have mapped the Rbt locus to an interval of approximately 6 cM on mouse Chromosome (Chr) 11 between microsatellite markers D11Mi t128 and D11Mit103. The Ts locus was mapped within a shorter interval of approximately 3 cM between D11Mit128 and D11Mit203. This indicates that Rbt and Ts may be allelic mutations. Sox9, the human homolog of w hich is responsible for the skeletal malformation syndrome campomelic dysplasia, was mapped proximal to D11Mit128. It is, therefore, unlikel y that Ts and Rbt are mouse models for this human skeletal disorder.