To determine the number of transcriptional isoforms for each gene, we examined the sim4 output for every matching cDNA in decreasing order of number of exons. The first cDNA was counted as one splice form, and for each subsequent cDNA, we determined whether exon structure was mutually exclusive to isoforms already counted. We were conservative in our definition of mutually exclusive, and thus our count represents the minimum number of isoforms represented in the cDNA collection.