To identify genes that are significantly enriched by mRNA tagging, we first normalized the total amount of Cy3 and Cy5 signal to each other in each hybridization. We measured the ratio of the signals from the co-immunoprecipitated mRNA (Cy5) to total RNA in the cell extract (Cy3), and calculated the percentile rank for each gene relative to all genes in each hybridization. The mean percentile rank was determined from eight repeats of the mRNA-tagging experiment. Student's t-test was used to determine which genes showed a mean enrichment significantly greater than the median enrichment for all genes (P<0.001).