Cas NS-1 is an acutely transforming murine retrovirus that induces early B-lineage lymphomas and occasional myeloid leukemias. The transforming sequence of this virus, v-cbl, shows no homology to known oncogenes but has some similarities to the yeast transcriptional factor GCN4. In this study we used a v-cbl probe to analyze mRNAs from a wide range of murine and human hemopoietic tumor cell lines and detected an 11-kilobase mRNA in all lineages. In normal mouse tissues the expression of c-cbl was highest in testis and thymus tissues, the predominant species in testis tissue being a 3.5-kilobase mRNA. The v-cbl oncogene was inserted into a bacterial expression vector to produce protein for the immunization of rabbits. Affinity-purified v-cbl antibodies identified abundant levels of p100gag-cbl in Cas NS-1-transformed fibroblasts and lower levels of a 135-kilodalton protein (p135c-cbl) in both normal and transformed cells. Subcellular fractionation showed that p100gag-cbl and p135c-cbl are both located in the nucleus and retained following 420 mM salt extraction. These results indicate that the translational product of a c-cbl is a 135-kilodalton nuclear protein.