Editor—Supernumerary marker chromosomes (SMCs) comprise a heterogeneous group of structurally arranged chromosomes. SMCs are found in approximately 0.14-0.72/1000 newborns1-3 and they may be associated with developmental abnormalities and malformations.4 The great variability of clinical symptoms in patients with SMCs is the result of the difference in the genetic content of the marker. The phenotypic consequences of SMCs are difficult to predict, especially if a de novo marker is detected prenatally. Therefore, the precise identification of a marker chromosome is of essential importance in genetic counselling. Earlier figures based on the results of a large prenatal multicentre study using conventional methods suggested a risk for an abnormal phenotype of 13% for SMCs.5 Combining FISH data and conventional analyses, the estimated risk for an abnormal phenotype turned out to be twice as high, approximately 28% for non-acrocentric autosomal SMCs and about 7% for acrocentric autosomal SMCs.6 An even more accurate method of identification of the chromosomal origin of supernumerary marker chromosomes is FISH with microdissection probes and reverse painting, allowing a definite delineation of phenotype-karyotype correlations.7

In recent years, a novel class of mitotically stable human marker chromosomes that are devoid of alpha satellite DNA has been identified.8 9 These analphoid markers have been shown to contain functional centromeres outside the normal centromere domain, which are called neocentromeres. Marker chromosomes derived from chromosome 5 are rare and a marker chromosome 5 with a neocentromere has not been reported so far.

We describe a patient with an inverted duplication of the distal part of the short arm of chromosome 5 and the formation of a neocentromere leading to a supernumerary marker chromosome. The comparison of the clinical findings of this patient with a tetrasomy of distal 5p with similar cases previously described suggests a gene …