The presence of 48 chromosomes in DS is a rare finding. It can be caused by an additional gonosome leading, e.g., to a karyotype 48,XXX,+21 [9,10] or by an sSMC [4]. Up to now, for sSMC derived from chromosomes 4, 7, 13 (two cases), 14, 14/22, 15 (two cases) and 22 (three case) have been reported in DS [4]. Ours is the first report of an sSMC in DS derived from chromosome 13 or 21 and the first complex rearrangement of one.

Among the complex rearranged of sSMC, chromosome 13/21 is the second most frequently involved [6]. An sSMC like the one we describe here, leading to partial trisomy 18p was reported in five individuals [6,9-11] in four of which no, or only minor malformations and/or mental impairment, were found [6,11,12], in the fifth, no postnatal data were available, but sonography gave normal results [13]. Apart from the pronounced retardation in motor development, we observed no obvious influence of partial trisomy 18p in our patient. This suggests that some genes present on 18p cause severe problems when present as four copies, as in i(18p)-syndrome [5], but not when present as three copies.

Competing Interests. The author(s) declare that they have no competing interests.