Mer is expressed in diagnostic samples derived from patients with B-ALL. Twenty-eight diagnostic bone marrow samples from patients with B-cell ALL were analyzed by (A) immunoblot and/or (B) flow cytometry for the expression of Mer protein. (A) Immunoblot 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; the complete data set is 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 one EP⁻ B-ALL sample (6034) and two EP⁺ B-ALL samples (612296 and 611729) are shown. All three samples expressed the CD19 B-lineage marker whereas only the EP⁺ samples exhibited Mer staining. (C) Mer expression was determined by flow cytometry in a prospective analysis of samples collected at diagnosis or relapse from patients with B-ALL. Each symbol represents data collected from a single sample. Approximately 32% of patients at diagnosis and 37% of patients at relapse express Mer on leukemic blasts (Mer positive + Mer dim).