A de novo direct duplication of 9p22-->p24 was shown in a child with a duplication 9p phenotype by GTG banding and fluorescence in situ hybridization (FISH) using a chromosome-9 specific painting probe as well as 6 YAC DNA probes localized to the 9p13-9p23 region. The breakpoints in this patient and previously reported patients suggest that 9p22 may be the critical region for duplication 9p syndrome.