High resolution banding analysis showed a de novo interstitial deletion, 46,XX, del(2) (q32.1q34), in a malformed and severely mentally retarded girl aged nine years. Biochemical studies showed that the proband had half normal activities of both erythrocyte isocitrate dehydrogenase (IDH1) and ribulose 5-phosphate 3-epimerase (RPE). It is suggested that the gene for RPE is located on the segment 2q32.1----q34.