Quantitative RT-PCR could serve as a clinical test of ZAP-70 mRNA expression. Real time quantitative RT-PCR was performed in 29 CLL samples representing the ZAP-70 mRNA expression spectrum defined by the DNA microarray analysis. ZAP-70 expression is shown relative to the expression of β-2-microglobulin in the same sample. The Pearson coefficient for correlation between the 2 methods was r = 0.901.