To mimic a typical clinical situation, in which small population of cancer cells are present among normal host cells in a primary tumor, we spiked decreasing amounts of total RNA extracted from VCaP cells into an excess of HeLa RNA, which does not have the fusion transcripts. The detection limit is 32 pg of VCaP RNA in the presence of 100 ng of HeLa RNA (Figure 2 ). This translates into only 1-3 cancer cells in the presence of 3000 times more normal cells. The level of sensitivity is superior to previous methods for detecting fusion transcripts (24) .