Simsek et al. (2005a) carried out a case-control study to determine the frequency of all DAT1-VNTR alleles in the Omani population and the degree of association between VNTR alleles and attention deficit hyperactivity disorder (ADHD). A total of 92 children diagnosed with ADHD according to DSM-4 criteria, and 110 healthy Omani blood donors as controls were selected for the study. VNTR alleles were amplified from DNA of both cases and controls by two independent PCRs to minimize genotyping errors, and separated on 2% agarose gel. Twelve DAT1 alleles were identified, three of which were the most common (10*10, 10*9, and 9*9). When comparing the frequencies of the genotypes observed in both the cases and control group with those by the Hardy-Weinberg distribution, no significant difference was found. No association between ADHD and DAT1-VNTR polymorphism in Oman could be demonstrated by the authors as the distribution of the DAT1*10 risk allele was found to be similar in both the cases (64.6%) and the controls (60.9%). Simsek et al. (2005a) surmised that this could have been due to the lack of detailed information on the control subjects. Among the control group, there were no gender differences in the DAT1*10 frequencies (males 60% and females 62%). On the other hand, a statistically insignificant frequency difference was found between males (69.4%) and females (55%) in the patient group. In another study, Simsek et al. (2005b) described the genetic variability of DAT 1 gene at the VNTR locus in 110 healthy Omani blood donors (60 males, 50 females). DNA was extracted, and the VNTR region of the DAT1 gene was amplified by two independent PCRs. Two common DAT1 alleles were detected - DAT1*10 and DAT1*9, as well as other five rare ones, *3,*6,*7,*8, and *11. Along with the three common observed genotypes*10/*10, *10/*9, and *9/*9 found in 45, 39, and 15 subjects, respectively, other nine rare genotypes were also detected. These included *3/*3,*6/*6, *9/*6, *9/*7, *10/*6, *10/*7, and *10/*8 each found in one subject, two subjects had *11/*10 genotype, and the other two had *11/*9 genotype. The observed heterozygous genotype at DAT1 gene in this study group (43.6%) was found to be less than that expected (51.7%). Frequency of DAT1*10 (60.9%) was found to be similar to the frequencies seen in Europeans and Africans, but lower than that of the Far East Asians, while that of DAT1*9 (33.2%) was similar to that observed in the French, Italians, and white Americans. The authors explained the finding of the relatively high frequency of DAT1*9 due to the interaction between the Omani and Yemeni (in whom DAT1*9 frequency is higher than DAT1*10 frequency) populations as well as with the East African populations. Certain rare alleles seen in this Omani population had earlier been noticed in other populations. These included DAT1*3 (African population), and DAT1*6 (present in Japanese, Italians, and Americans, but absent in closely related Yemenite Jews and Druze populations).

In 2006, Simsek and colleagues developed a new polymerase chain reaction (PCR) test that provides an improved detection of the VNTR alleles in the human DAT1 gene. Using two independent PCR methods, Simsek et al. (2006) determined the distribution of VNTR alleles in 110 healthy Omani subjects, and in 92 children with attention-deficit hyperactivity disorder (ADHD). The frequency of the risk allele (DAT1*10) was similar in the healthy subjects and ADHD cases, indicating absence of association of this allele with ADHD in Oman.

El-Tarras et al. (2012) studied the polymorphisms in dopamine related genes and their association with ADHD in Saudi children.  A total of 120 ADHD diagnosed children were recruited for the study along with 160 healthy age-matched controls.  The 40-bp Variable Number Tandem Repeats (VNTR) in the 3’ Untranslated Region (UTR) of the DAT1 gene was analyzed.  Four alleles were identified, namely, 7-repeat, 9-repeat, 10-repeat and 11-repeat, giving rise to 6 genotypes: 10/10, 10/9,11/11, 11/7, 11/10 and 10/7.  The allele frequency and genotype frequency were compared between ADHD cases and controls.  The 11-repeat allele was found to be most prevalent amongst cases while allele 10 was prevalent in controls.  It was found that two alleles i.e. 7 and 11 repeats, were found to be significantly associated with the risk of ADHD (p<0.01, OR=2.5, 3.3).  The genotypes 11/11 and 11/7 showed a similar significant association with the disease (p<0.01, OR=4, 3).  The alleles of 9 and 10 repeats were found to be protective against ADHD (p<0.01, OR=0.4).  The genotypes 10/10 and 10/9 showed a similar protective effect (p<0.01, OR=0.3).  The authors noted that the study may be limited by a small sample size.