Southern blot analysis reveals 2 types of 5′ Notch1 genomic rearrangements in murine cell lines expressing type 1 (I) or type 2 (II) aberrant transcripts. Genomic DNA (10 μg) from the indicated cell lines was digested with Eco R1 and analyzed on Southern blots. Panels A, B, and C correspond to Southern blots hybridized to probes A, B, and C, which are homologous to the regions shown in the diagram of the Notch1 locus above the blots. The positions of genomic Eco R1 sites (R1) around the Notch1 locus are expressed relative to the position of the ATG start codon in exon 1, which is designated position 1. The positions of murine-specific alternative 5′ Notch1 exons 1a, 1b, and 1c are also shown. R indicates rearranged; and GL, normal genomic locus.