Editor—Five of the human autosomes are acrocentric, chromosomes 13, 14, 15, 21, and 22, and are identified by the presence of satellited short arms. These short arms contain three bands, p11, p12, and p13,1 and are composed of repetitive DNA containing satellite repeats and copies of ribosomal RNA genes. Band p11, the pericentromeric region, is composed of several types of tandemly repeated DNA including satellites I, II, III, and IV, and β satellite DNA.2-4 Band p12, the satellite stalks, contains multiple copies of genes coding for ribosomal RNA5 and is known as the nucleolar organiser region (NOR) as the nucleolus is formed by an aggregation of ribosomal RNA. This can be recognised by staining with silver nitrate (AgNOR staining).6 Band p13 contains β satellite DNA and terminal telomeric sequences.2 7

Loss or gain of the short arm of acrocentric chromosomes occurs without apparent phenotypic effect. For example, Robertsonian translocations occur when two acrocentric chromosomes are joined by centric fusion with the resulting loss of the short arm material and have no associated phenotype in this euchromatically balanced form.8 Chromosomal rearrangements involving the short arms of acrocentric chromosomes are a well known form of chromosomal variation. The most common variation results from rearrangements between the short arms of acrocentric chromosomes. Thus, the satellites of acrocentric chromosomes range in size from no satellites to double or treble satellites as shown by AgNOR staining.8Translocations between the short arm of an acrocentric chromosome and the heterochromatic region of the long arm of the Y chromosome, resulting in acrocentric chromosomes with Y chromosome heterochromatin in place of satellites and satellited Y chromosomes, are also observed.9 10 More rarely, non-acrocentric chromosomes with terminal satellites have been described which arise from a translocation between the short …