A deazaguanine-substituted DNA PCR product from FMR-1 (the fragile X mental retardation syndrome gene) can be efficiently visualized with ethidium bromide on standard agarose gels. Normal-sized alleles (less than 54 CGG repeats) generated strong, easily visible bands in the expected size range of 491-635 bp. Southern blot analysis and radioactive PCR on sequencing gels were used to verify that the 74 males (out of 245 total tested) whose DNA failed to generate a visible band contained premutations or full mutations. This technique can be used as an inexpensive screen for fragile X syndrome among developmentally delayed males.