Sensitivity for detection of bcr-abl–rearranged mRNA. Products from RT-PCR amplification were visualized by ethidium staining after agarose gel electrophoresis. Lane 1, 100-nt length marker; lane 2, positive control (cell line K562); lane 3, negative control (healthy donor); lanes 4 through 8, serial dilution of K562 cells (1:10¹, 1:10², 1:10³, 1:10⁴, and 1:10⁵, respectively) into cells from a healthy donor. Down to 1 K562 cell diluted in 10⁴ donor cells is easily detectable.