Lateral spine radiograph of the mother (III,6) showed a bilateral break in the pars interarticularis (lucency shown by arrow head). The latter was the reason to develop forward slippage of L5 on the S1; she is a heterozygous carrier of disease causing mutations in the ARSB gene as well as in the GNS gene, which denote an impaired lysosomal degradation of dermatan sulfate and heparan sulfate in the fully developed diseases MPS VI and MPS IIID.