The Infinium Methylation Assay uses two different bead types to detect CpG methylation. The U bead type matches the unmethylated CpG site; the M bead type matches the methylated site. In the top figure, the unmethylated CpG target site matches with the U probe, enabling single-base extension and detection. It has a single-base mismatch to the M probe, which inhibits extension. If the CpG locus of interest is methylated (bottom figure), the reverse occurs.