REGION-SPECIFIC FISH PROBES USED TO IDENTIFY AND CHARACTERIZE AN INTERSTITIAL PARACENTRIC INV(21)(Q22.1Q22.3)

Region-specific probes, developed for the diagnosis of specific syndro mes, can be adapted to elucidate the exact nature of certain chromosom al structural anomalies. We describe the use of FISH probes in charact erizing a prenatally diagnosed chromosome rearrangement. An abnormal c hromosome 21 was detected during amniocentesis for maternal age indica tion, and a similar appearing chromosome 21 was found in the mother. T he exact nature of the rearrangement was not immediately evident from G-banded karyotypes. FISH was performed using a whole chromosome paint ing probe, as well as the region-specific probes D21S65 (21q21-22.1), D21S55 (21q22.3) and D21S1219/D21S1220 (21q22.3-qter) (Oncor). Results showed an interstitial paracentric inversion, with breakpoints in ban ds 21q22.1 and 21q22.3, which was identical in the mother and the fetu s: 46,XX,?inv(21)(q).ish inv(21)(q22.1q22.3)(wcp +,D21S65 mv,D21S55 mv ,D21S1219/D21S1220 st). In this case, FISH using region-specific probe s was helpful in characterizing the inversion and aided in the genetic counselling of risk assessment for the family. (C) 1998 John Wiley & Sons, Ltd.