**3.3 OXTR genotyping**

Besides the OXTR gene, one study examined genes linked with monoaminergic neurotransmission, such as DR1, COMT, and SLC6A4 [16]; while another explored the CD38 [19], a gene implicated in oxytocin release. For the OXTR gene, polymorphisms rs53576, rs237897, rs1042778, and rs2254298 were studied, being rs53576 analyzed in all studies. Only one study explored the interactions between different OXTR SNPs and although they did not find any significant effect on ERP components, they did on social cognition measures [17].

The genotypic frequencies for the rs53576 variants across the reviewed studies are shown in **Figure 2**. Some studies did not report the genotypic or allelic frequencies [19, 20, 23], so we calculated these values from their HWE report. The three studies conducted with Asian participants revealed lower GG genotype frequencies, while two of the five studies with Caucasian participants found a greater ratio of GG homozygotes. The three remaining studies did not satisfy the HWE probe, pointing to a large proportion of heterozygotes (**Figure 2**).
