BackgroundAutosomal monosomies in human are generally suggested to be incompatible with life; however, there is quite a number of cytogenetic reports describing full monosomy of one chromosome 21 in live born children. Here, we report a cytogenetically similar case associated with congenital malformation including mental retardation, motor development delay, craniofacial dysmorphism and skeletal abnormalities.ResultsInitially, a full monosomy of chromosome 21 was suspected as only 45 chromosomes were present. However, molecular cytogenetics revealed a de novo unbalanced translocation with a der(7)t(7;21). It turned out that the translocated part of chromosome 21 produced GTG-banding patterns similar to original ones of chromosome 7. The final karyotype was described as 45,XX,der(7)t(7;21)(q34;q22.13),-21. As a meta analysis revealed that clusters of the olfactory receptor gene family (ORF) are located in these breakpoint regions, an involvement of OFR in the rearrangement formation is discussed here.ConclusionThe described clinical phenotype is comparable to previously described cases with ring chromosome 21, and a number of cases with del(7)(q34). Thus, at least a certain percentage, if not all full monosomy of chromosome 21 in live-borns are cases of unbalanced translocations involving chromosome 21.