Microarrays provide a means of studying expression level of tens of thousands of genes by providing one or more oligonucleotide probe(s) for each transcript studied. Affymetrix® GeneChip™ platforms historically pair each 25-base perfect match (PM) probe with a mismatch probe (MM) differing by a complementary base located in the 13th position to quantify and deflate effects of cross-hybridization. Analytical routines for analyzing these arrays take into account difference in expression levels of MM and PM probes to determine which ones are useful for further study. If a single nucleotide polymorphism (SNP) occurs at the 13th base, a probe with a higher MM expression level may be incorrectly omitted. In order to examine SNP affects on PM and MM expression levels, known human SNPs from dbSNP were mapped to probe sets within the Affymetrix® HG-U133A platform. Probe sets containing one or more probe pairs with a single SNP at the 13th position were extracted. A set of twelve microarray experiments were analyzed for the PM and MM expression levels for these probe sets. Over 6,000,000 human SNPs and their flanking regions were extracted from dbSNP. These sequences were aligned against each of the 247,965 probe pair sequences from the Affymetrix® HG-U133A platform. A total of 915 probe sets containing a single probe sequence with a SNP mapped to the 13th base were extracted. A subset containing 166 probe sets result in complementary base SNPs. Comparison of gene expression levels for the SNP to non-SNP PM and MM probes does not yield a significant difference using χ2 analysis. Thus, omission of probes with MM expression levels higher than PM expression levels does not appear to result in a loss of information concerning SNPs for these regions.