We report on a 8-year-old patient affected by a selective T-cell defect associated with mental retardation and dysmorphic signs. At birth thymic aplasia and hypoparathyroidism were noted, suggesting a DiGeorge-like anomaly. The immunological evaluation during the 8 years follow-up revealed a progressive decrease of CD3+CD4+ lymphocytes, which paralleled deficiencies of blood T cells. Chromosome analysis using GTL banding revealed an interstitial deletion of the short arm of chromosome 10. We next investigated whether the expression of IL-2R alpha chain and Nil-2-a genes, which are located on the short arm of chromosome 10, was affected by the deletion. Transcription of these two genes was normal, thus suggesting that the two regions were preserved. In situ hybridization studies with the painting libraries #G3A7 and #G9 confirmed that the two regions were preserved and allowed us to define the breakpoint as 10p12-10p13. Due to the similarities between DiGeorge and 10p syndromes, we suggest that the 10p13-10p12 region contains a gene(s) potentially related to gene products of the 22q11 region, frequently altered in patients with DiGeorge.