Real-time PCR analysis of selected gene transcripts observed to be differentially expressed in RNA expression profiling experiments. Thirteen gene transcripts that were observed by microarray analysis to be differentially expressed at least 2-fold were chosen and tested. However, by microarray analysis, it was not possible to conclude which of the 2 isoforms of the HMBS gene (erythroid-specific and housekeeping isoforms) was differentially expressed because the probes for this gene on the array chip did not discriminate between the 2 isoforms.²³  Therefore, real-time PCR was performed using primers specific for each of the 2 different isoforms of HMBS. Real-time PCR results confirmed that all 13 gene transcripts (including the 2 isoforms of HMBS) were indeed differentially expressed between KLS-EML-GFP and KLS-EML-HOXB4 cells, as observed in our RNA expression profiling experiments. Interestingly, the erythroid-specific isoform of HMBS was observed to be down-regulated approximately 28-fold more than the housekeeping isoform of HMBS (61.2-fold vs 2.2-fold). Values are mean plus or minus SD of triplicate PCR analyses of RNA from 3 different single-cell clones of each cell type.