Anecdotal reports link a distinct, bilateral, parieto-temporally generated 4.5-Hz rhythm on an electroencephalogram to a methylenetetrahydrofolate reductase gene variant co-occurring in autism spectrum disorder, but the validation of its precision is needed. The electroencephalograms of children with autism spectrum disorder showing the distinct bilateral parieto-temporally generated 4.5-Hz rhythm and their clinical chart report on polymerase chain reaction screening for methylenetetrahydrofolate reductase gene variants, "677C>T" and "1298A>C," were retrieved from an outpatient clinic between February 2019 and April 2024. Twenty-five cases were identified. Patients were between 2 and 12 (7 ± 3) years old from Asian (n = 16, 64%), European (n = 5, 20%), African (n = 1, 4%) and mixed (n = 3, 12%) ethnicities. Twenty patients (80%) were positive for 677 C>T[subscript heterozygous] (n = 3, 15%), 1298A>C[subscript heterozygous] (n = 8, 40%) or both (n = 9, 45%). The polymerase chain reaction testing detected neither variant in 5 (20%) patients. Therefore, the electroencephalogram-endophenotype showed 80% precision in identifying methylenetetrahydrofolate reductase gene variant within the sample. This preliminary data support the precision of the proposed distinct, bilateral, parieto-temporally generated 4.5-Hz rhythm in identifying methylenetetrahydrofolate reductase gene variants and its potential clinical applications as a valuable, non-invasive and objective measure within the population.