(A) Fluorescence image of a small portion of an oligonucleotide tag array following hybridization of 77 labeled SBE primers. The entire array contains >32,000 20-mer tag probe pairs. The physically adjacent PM and MM probes for a single tag probe pair are labeled. (B) The fluorescence intensity pattern for a tag probe pair showing the presence of an AA homozygote, an AB heterozygote, and a BB homozygote, and the computed relative allele fraction value P = (PM−MM)_fluorescein / [(PM−MM)_fluorescein + (PM−MM)_{phycoerythrin}]. Because of the partial overlap of the emission spectra of fluorescein and phycoerythrin, there is some spillover of fluorescein signal into the phycoerythrin emission channel. Background signals are subtracted and corrections for spectral overlap are applied prior to the quantitative genotyping analysis.