Potential mechanisms for the reducing BRCA1 expression. Schematics of the two BRCA1 alleles are shown. (a) Endo-reduplication of a methylated allele would lead to complete loss of expression. (b) Expression from only one allele could be indicative of methylation of one of the promoters. (c) Methylation of both promoters through an unidentified mechanism would lead to a dramatic decrease in expression. (d) Increased NBR2 expression could lead to decreased BRCA1 expression. (e) p53 could inhibit factors such as E2F. (f) Loss of DNA binding by the GABP-α/β complex leads to decreased expression. (g) ID4 expression leads to the down regulation of the BRCA1 promoter, presumably through interaction with a transcription factor that normally regulates the promoter. (h) BRCA1 and ID4 levels appear to be coregulated.