Mer is expressed in diagnostic samples derived from patients with E2A-PBX1⁺ B-ALL. Twenty-eight diagnostic bone marrow samples from patients with B-cell ALL were analyzed by Western blot (A) and/or flow cytometry (B) for the expression of Mer protein. A subset of these samples was also analyzed by quantitative real-time RT-PCR (C) for the presence of Mer mRNA transcript. (A) Western blot analysis of Mer protein (∼ 180 kDa) in representative samples of non–E2A-PBX1 (EP⁻) B-ALL (lanes 1-5) and E2A-PBX1⁺ (EP⁺) B-ALL (lanes 6-8). Patient sample ID is shown in parentheses for comparison to the complete dataset listed in supplemental Table 1. The membrane was stripped and reprobed with anti-Actin (∼ 43 kDa) antibody to confirm similar loading of total protein. (B) Representative flow cytometry profiles for 1 EP⁻ B-ALL sample (6034) and 2 EP⁺ B-ALL samples (612296 and 611729) are shown. All 3 samples expressed the CD19 B-lineage marker, whereas only the EP⁺ samples exhibited Mer staining. (C) Quantitative real time RT-PCR for 2 representative EP⁺ B-ALL samples exhibiting 100- to 200-fold increase in Mer transcript relative to the mean expression in 5 EP⁻ B-ALL samples which express negligible amounts of Mer mRNA. Mean values and standard errors were derived from 3 independent experiments performed in duplicate.