The structure of the human and mouse BRCA1 loci are shown. Transcription initiation sites are indicated by arrows. The general exon structure is indicated by the variously shaded boxes. The patterned box indicates the location of an acidic ribosomal phosphoprotein P1 pseudocopy , which appears to have been inserted into the region between NBR1 and the BRCA1 pseudo-gene. The BRCA1 gene appears to have two alternative first exons, although the 1A exon appears to be used much more frequently.