Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) is a transferase enzyme that catalyzes conversion of hypoxanthine into inosine monophosphate (IMP); and guanine to guanosine monophosphate (GMP) (1). This enzyme provide cells with alternatives to the energy-expensive de novo synthesis of nucleotides and plays a critical role in the maintenance of intracellular purine nucleotide pools in cells that have decreased ability to synthesize new nucleotides (e.g. erythrocytes) (2). A deficiency of HGPRT in males results in a spectrum of disease, the severity of which is dependent upon the extent of the deficiency. A complete deficiency of the enzyme is associated with the Lesch-Nyhan syndrome (LNS) (incidences of 1 in 100,000), while partial deficiency is associated with gout (incidences 1 in 200 among males with gout) (3). LNS was first described by M. Lesch and W. Nyhan in 1964 as a complete deficiency of activity reported in two brothers (4) and clinically associated with neurologic dysfunction, cognitive and behavioral disturbances including self-mutilation, and uric acid overproduction (hyperuricemia). Most patients with LNS usually expire in childhood from renal failure and infection; and survival into second decade of life is known but rare (5). Sudden and unexpected deaths have respiratory rather than cardiogenic origins (6). Neurological symptoms affect the motor sphere and the cognitive and behavioral aspects (7). HGPRT is encoded in humans via the HPRT1 gene (size 42.5 kbp) (8) on the X chromosome in the region q26-q27 with only one transcript (9) and consists of 9 exons with a coding sequence of 654 bp (10). There are high degrees of heterogeneity in type and location within the gene (7). At least 400 different mutations in the HPRT coding parts of exons 1 to 9 are specified and mutations of substitution, frame shift, micro deletion, macro deletion, insertion, and splicing errors have been described as the cause of HGPRT deficiency (11, 12). However, about 30% of patients’ mothers are not somatic carriers; these patients probably carry de novo mutations due to a germinal cell mutation that requires genomic DNA sequencing (7). Here, we report a new small duplication that was detected by sequencing the entire HPRT1 gene in an Iranian family.