Supernumerary der(1) marker chromosome derived from a ring chromosome 1 which has retained the original centromere and euchromatin from 1q21.1 --> q21.3 with substantial loss of 1q12 heterochromatin in a female with dysmorphic features and psychomotoric developmental delay.

We report on a 5.5-year-old girl with dysmorphic features and psychomotoric developmental delay with a mitotically stable supernumerary marker chromosome. The origin of the marker was identified by microdissection and reverse painting of marker DNA as the pericentromeric region of chromosome 1. Fine mapping by FISH with selected YAC or BAC clones identified no p-arm material on the marker. The marker has retained its original centromere and euchromatin from 1q21.1-q21.3 but only small remnants of the 1q12 heterochromatin. Furthermore, some FISH clones presented single signals on the marker and others presented double signals indicating a partial duplication within the marker. These observations suggest a multi-step origin of the marker most probably with ring formation as the first step.