The euchromatic 9p+ polymorphism is a locus-specific amplification caused by repeated copies of a small DNA segment mapping within 9p12.

A large duplication involving the proximal euchromatic region of chromosome 9p was detected by conventional cytogenetics in a healthy 33-year-old woman and in two unrelated foetuses; both of them received the rearrangement from their healthy father. The duplicated segment was R(RBG) and C(CBG)-negative and G(GTG)-positive and was also positive for a 9-specific painting probe. It was preliminarily interpreted as a pathological quantitative change of the genome in the foetuses. FISH analyses allowed us to characterise the chromosome boundaries of this polymorphism, being identified by the RP11-15E1 BAC clone, proximally, and by the RP11-402N8 clone, distally, both probes falling within the 9p12 region. The contiguous, distally, RP11-916H19 probe was not included in the amplification, and may represent the discriminating genetic locus between chromosome polymorphism and chromosome mutation. The 9p12 amplification was approximately 12, 7 and 8 Mb in the three different families and was stable through generations. Our observations confirm the already provided evidence that proximal 9p duplications represent a benign euchromatic polymorphism. However, we demonstrated that these variants are not a simple duplication of the region 9p11.2-p13.1, as already suggested, but that they result from a many-fold amplification of a segment mapping within 9p12. These results provide important insights both in the genetic counselling and in the prenatal diagnosis of rare euchromatic chromosome variants and in understanding the architecture of the human genome.