Sirenomelia with a de novo balanced translocation 46,X,t(X;16)(p11.23;p12.3).

We report a female fetus with sirenomelia with 46,X,t(X;16)(p11.23;p12.3) de novo. Fluorescence in situ hybridization (FISH) with bacterial artificial chromosomes were employed for narrowing down the breakpoint regions. On chromosome 16, the breakpoint was mapped in the region of RP11-453F10 (19 920 640-20 118 153 bp from 16pter). On chromosome X, the breakpoint was mapped in the region of RP11-794A15 (47 333 744-47 524 066 bp from Xpter). This is the first case report of sirenomelia associated with translocations. The abnormal phenotype, associated with a balanced translocation, was caused by deletion or breakage of dosage-sensitive genes of the breakpoint, disruption of an imprinted gene, or uniparental disomy. Although the parental origin of normal 16 and der(16) remained undetermined, this case will provide insight into the pathogenetic mechanism of sirenomelia.