Presence of 3′ Notch1 transcripts and 5′ genomic deletions in primary Ik^L/L tumors. The indicated RNA or DNA samples were analyzed for the presence of exon 27b–containing transcripts by RT-PCR (top) and genomic deletion of sequences between 2 recombination signal sequences (RSS) present in the 5′ region of the Notch1 locus by PCR (bottom). RAG-mediated deletion of the sequences between the RSS sites moves the sequences of the PCR primers P1 and P2 closer together, allowing amplification of a 500-bp fragment. The Ik^L/L sample was extracted from premalignant thymocytes of a 7-week-old mouse; T7 and T29 are cell lines derived from IN1C⁻ and Ik^L/L tumors, respectively; T49 and T90 are the primary tumors from Figure 5A. β-Actin and exon 34 of Notch1 served as control RT-PCR and PCR reactions, respectively. *Products that may correspond to splicing intermediates of transcripts initiated upstream.