OBJECTIVETo identify the origin of human small supernumerary marker chromosomes (sSMCs) using fluorescent in situ hybridization (FISH) combined with G-banding karyotype analysis, and to discuss their mechanisms of formation and research value.

METHODSCep-FISH and SubcenM-FISH were used to analyze sSMCs in 3 patients for whom the result of G-banding was 47,XN,+mar.

RESULTSThe FISH result of case 1 was 47,XY,+mar.ish inv dup(22)(q11.1)(D22Z4++,D14/22Z1+, RP11-172D7-). The marker has formed exclusively by heterochromatin. A boy was delivered later with no apparent clinical abnormalities. The FISH result of case 2 was 47,XX,+mar.ish r(10)(p11.2q11.2) (cep10+, RP11-232C13+, RP11-178A10+)[25]/46,XX[10]. The marker has formed by heterochromatin and nearby centromere. A girl was delivered later with no clinical abnormalities. The FISH result of case 3 was 47,XY,+mar.ish inv dup(22)(q11.1)(D22Z4+,D14/22Z1+). The marker has also formed exclusively by euchromatin. Fetal abnormalities were detected by type B ultrasonography, but were not necessarily related with the marker.

CONCLUSIONThe diversity of sSMCs has posed a great challenge for prenatal diagnosis. Identification of sSMCs will require combined karyotype analysis and FISH or other molecular techniques such as microarray based comparative genomic hybridization or sequencing. For its specific structure, the sSMCs may also provide a valuable tool for gene mapping, heterochromatin research and gene therapy.