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We studied a female patient initially diagnosed with Costello syndrome who carries an apparently balanced translocation, t(1;22) (q24.3;q13.1). Molecular characterization of the translocation revealed a mosaic of two derivative chromosomes 1 in her peripheral blood lymphocytes, in one of which the coding region of the platelet-derived growth factor (PDGFB; chromosome 22q13.1) gene was disrupted. Both the initial translocation and the secondary intrachromosomal rearrangement appear to have occurred by nonhomologous (illegitimate) recombination. In 18 patients with Costello syndrome, mutation analysis of the genes belonging to the PDGF/R family, PDGFA, PDGFB, PDGFC, PDGFD, PDGFRA, and PDGFRB, revealed no pathogenic mutations. Reevaluation of the clinical symptoms of the translocation patient challenges the diagnosis of Costello syndrome in this patient. In total RNA isolated from lymphocytes of the translocation patient, we identified four different fusion transcripts consisting of PDGFB exons and parts of chromosome 1q24.3. In two of the mRNAs, exon 6 of PDGFB, encoding the 41 C-terminal amino acid residues, was absent. Immunofluorescence analysis showed that the wild-type protein was dispersed and formed a network-like structure in the extracellular matrix, whereas the two aberrant PDGFB proteins were localized in aggregates. We speculate that the biological consequences of the mutant PDGFB allele contributed to the unique disease phenotype of the translocation patient.