The chromosomal localization of a unique DNA fragment, closely linked to Huntington disease (HD), was assessed in situ by hybridization with 2-acetylaminofluorene (AAF) modified probes. In these experiments, a cosmid cloned genomic fragment (c5.5) was used for hybridization. Here we present evidence that confirms the mapping of the D4S10 locus to the p16 region of chromosome 4 and assigns it to the telomere of the short arm.