Sensitivity of E2A-PBX1 PCR assay. TheE2A-PBX1 fusion mRNA and the relative location of amplification primers (arrows) and detection oligonucleotides (lines) are schematically depicted at the top. Below this are displayed a photograph of an ethidium bromide-stained agarose gel of nested PCR products and a Southern blot of this gel hybridized with radio-labeled Probe-fus oligonucleotide (identical results were seen with Probe-PBX1). Molecular size (MW) markers are indicated at left in base pairs. Samples include t(1;19)⁺ RCH-ACV and t(1;19)⁻ REH pre-B ALL cell lines, serial 10-fold dilutions (10⁻¹ to 10⁻⁶) of RCH-ACV RNA into REH RNA, and reverse transcriptase (RT) and PCR negative (NEG) controls.