Evidence for different mechanisms of loss of heterozygosity on chromosome 1. (A-C) Dual-color fluorescent in situ hybridization signal patterns of selected cell nuclei obtained with PAC clones that encompass the RHD/RHCE (FITC, green) and, as a control, AF1q gene sequences (Cy3, red). (A) As shown here for patient 9, both the segmented (top) and round (bottom) nuclei in 4 of 6 analyzed cases contained 2 signals each and, thus, 2 RH gene loci. (B) The lack of one green signal in the majority of segmented nuclei and a minority of round nuclei of patient 3, on the other hand, confirmed a monoallelic deletion of at least the RHD/RHCE gene locus. (C) The 2 green RH and 3 red AF1q signal pattern in patient 6 prompted further investigations that (D) confirmed the presence of 3 chromosome 1 (green) and 2 chromosome 7 (red) centromeres with one colocalization (orange). This pattern reflects the presence of a der (1;7)(q10;p10), a highly specific chromosome abnormality in particular subtypes of myeloid malignancies. Thin lines separate individual images of representative cell nuclei. Signals were recorded after overnight incubation at 37°C. Images were visualized using a Zeiss Axioplan microscope equipped with 100×/1.45 alpha Plan-Fluar lens (Carl Zeiss, Heidelberg, Germany). Images were acquired using a Photometrics (Tucson, AZ) charge-coupled device camera, and IPLAB software (VYSIS, Stuttgart, Germany), and were processed using PowerPoint (Microsoft, Redmond, WA). Original magnification × 1000. Thin lines separate individual images of representative nuclei.