An unusual reciprocal translocation detected by subtelomeric FISH: interstitial and not terminal.

An 11-month-old boy with a pattern of dysmorphic signs, an atrial septal defect, right inguinal hernia, bilateral undescended testes, bilateral urinary reflux, right renal dysplasia, and developmental delay had an abnormal chromosome 11 with additional material of unknown origin attached to the long arm in his karyotype. The paternal karyotype was normal 46,XY, while the mother's karyotype was 46,XX,t(2;11)(q35;q24.2). Thus, a reciprocal terminal exchange was assumed resulting in duplication of distal 2q material and a small subterminal 11q deletion. However, application of subtelomeric fluorescence in situ hybridization (FISH) probes indicated that the translocation was not a terminal reciprocal exchange, but was interstitial at least for one of the chromosomes, which would be highly unusual since most interstitial translocations are non-reciprocal. Based on the results of FISH and microsatellite marker examinations, the designation of the breakpoints and thus of the deleted and duplicated segments had to be revised. The findings have implications for karyotype-phenotype correlation.