Increasing matrix complexity further, we explored the FIT probes in the flow cytometry-based detection of TCR mRNA in living T cells. To deliver the probes into these "hard-to-transfect" cells, we optimized microporation and found that the majority of the cells (60-80 % of 10 6 cells, Fig. ) survived a 25 ms long, single 240 V pulse, while allowing relatively weak staining. We then set out to distinguish Jurkat T cells from CCRF CEM cells, which are comparable in size and granularity. In initial experiments, we microporated "traditional" FIT probes, which contained only QB dyes (data not shown). However, their fluorescence intensity was too low for flow cytometric detection of TCR mRNA in CEM cells and Jurkat cells. With the brighter light harvesting probes it was possible to microporate with concentrations between 100 and 10 nM and still have fluorescence signals above background. For example, flow cytometric analysis of 6 replicates (see Fig. for a representative example) showed that delivery of JK2 increased the mean fluorescence intensity of Jurkat cells by an average of 177 % (Table ), while the fluorescence intensity of CCRF CEM cells remained closer to background (+ 68%). Over the 6 replicates, a population of 26 % of the 6 x 10.000 -15.000 live Jurkat cells analyzed showed higher than background fluorescence, compared to only 12 % of the CCRF CEM cells (see Fig. for a representative example and Fig. for averaged data). In probes JK1, JK3 and JK4 we varied the position of the auxiliary dyes. After microporation, Jurkat cells gave again higher fluorescence than CCRF CEM cells (Fig. ).