Expression of flt3R-specific mRNA by MSCV-FL leukemic cells. Northern blot analysis of total cellular RNA (10 μg) prepared from leukemic spleens (SPL) of secondary MSCV-FL recipients 196, 199, 203, and 204 or from corresponding liquid cultures of leukemic cells maintained in the presence of KL, IL-3, and IL-7 plus G418 for 2 to 3 weeks. The blot was sequentially hybridized with probes specific forneo (3.7, 3.0, and 1.3 kb), FL (3.7 and 3.0 kb), flt3R (3.2 kb), and lysozyme (1.7 kb) transcripts. The relative amounts of RNA loaded are indicated by hybridization to a probe specific for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) sequences (1.4 kb). BALB/c SPL, negative control. Sizes were determined by comparison to 28S and 18S rRNAs (4.5 and 1.8 kb, respectively).