Deletion of the 5′ Notch1 promoter activates cryptic 3′ internal promoters to allow expression of truncated Notch1 proteins lacking the ectodomain. (A) Schematic depiction of full-length wild-type and mutant Notch1 proteins. The ectodomain includes the EGF repeats and the NRR, which contains 3 Lin/Notch repeats (LNR) and the heterodimerization domain (HD). The Notch1 intracellular region (ICN1) contains a C-terminal PEST domain (P). Notch1 transcripts lacking most of the ectodomain are translated using a conserved internal initiator methionine (M1727) that is just upstream of the GS cleavage site in the TM region. In T-ALLs characterized by both groups, these truncated Notch1 proteins also frequently exhibited PEST domain truncations within ICN1. (B) Spontaneous or targeted Notch1 deletions that include the 5′ promoter and transcriptional start site (TSS) in exon 1 allow transcription of truncated Notch1 mRNAs that initiate in exons 25-27 (indicated by arrow). Ikaros loss facilitates epigenetic remodeling to permit transcription from 3′ cryptic promoters in this region. (Professional illustration by A. Y. Chen.)