Achondrogenesis type IB (ACG-IB) is a recessively inherited chondrodysplasia characterized by extremely poor skeletal development and perinatal death^1–6. A defect in sulphate metabolism leading to reduced sulphation of proteoglycans has been reported in one case⁷. The gene for diastrophic dysplasia (DID), a milder chondrodysplasia, was recently shown to encode a sulphate transporter (DTDST)⁸. We set out to test whether mutations in DTDST might be the cause of ACG-IB using cell cultures, cartilage samples and DMA from six patients. We found that ACGIB cartilage contains less sulphate than control cartilage, and that a sulphation defect is present in all ACG-IB cells and is caused by impaired sulphate uptake. Seven DTDST mutations accounting for all twelve chromosomes were identified in six ACG-IB patients. Thus, ACG-IB and DTD are allelic disorders. While DTD is associated with reduced DTDST expression, ACG-IB is produced by homozy-gosity or compound heterozygos-ity for structural mutations predicting little or no residual activity of the sulphate transporter and represents the null phenotype of DTDST.