The recent advent of exome sequencing has allowed for the identification of pathogenic gene variants responsible for a variety of diseases that were previously clinically diagnosed, with no underlying molecular etiology. Among these conditions, intellectual disability is a prevalent heterogeneous condition, presenting itself in a large spectrum of intensity, in some cases associated with congenital malformations, behavioral and various other intellectual development alterations. Here we report on a 36-year-old male patient, with a mild intellectual disability that remained undiagnosed at the molecular level for all his life. Using Nextera Exome Sequencing, a Chr3:9.517.294 A>AC (c.3848_3849insC) SETD5 gene insertion was found. This rare variant was classified as likely pathogenic due to its frameshift nature in the gene, in which loss-of-function mutations have been previously reported to cause intellectual disability, as well as a 3p25.3 microdeletion phenotype. It is possible that this variant shows partial activity, due to its gene localization, which would explain the patient’s mild phenotype when compared with other reports.