Distinguishing CCRF CEM cells from Jurkat cells based on the fluorescence of CCRF CEM-specific probes was more challenging. We screened probes (Table ) that differed in labelling positions and targeted segments within CDR3 of TCR mRNA. Probes C6 and C14 enhanced the mean fluorescence intensity of live CCRF CEM cells by 58 and 120%, respectively, compared to 25% and 67% of live Jurkat cells (Table ). For example, histograms of CCRF CEM cells microporated with C14 consistently showed a broadening to a population of cells that emitted intense fluorescence in a greater number of cells (see Fig. ). On average, 15 % of the CCRF CEM cells had higher fluorescence than non-microporated cells, whereas only 6 % of the Jurkat cells exceeded background fluorescence (Fig. ). Superimposition of representative histograms (Fig. ) indicates that the shift towards higher fluorescing cells is indeed more pronounced in CCRF CEM cells.