Small supernumerary marker chromosomes (SMC) are reported in 0.01-0.05% of all prenatally screened fetuses, approximately half of which are cases with inv dup(15). Amongst the remaining cases there is (i) a great variation in chromosomal and parental origin, (ii) the possibility of genomic imprinting effects, and (iii) homozygosity of autosomal recessively inherited mutations in uniparental isodisomy [1,2]. Since there are great variations in their clinical outcomes, the characterization of prenatally de­tected, particularly de novo SMC, is of significant interest that more appropriate medical care and genetic counseling for the parents can be provided.

Characterization of one or more SMC in a patient should be followed by testing for uniparental disomy (UPD), as UPD can also cause clinical signs and symp­toms [3-5]. In the present paper we suggest a strategy for the fast, straightforward and comprehensive characteriza­tion of small SMC by molecular cytogenetic and molecu­lar methods.