Carrying a mutation in one copy of the BRCA1 gene leads to increased risk of developing breast and/or ovarian cancer because as a woman ages, she risks mutating the other copy of the BRCA1 gene. Surprisingly, analysis of some cancers in BRCA1 carriers revealed one mutant copy and one normal copy of BRCA1 at the sequence level. Choose from below the most likely explanation for this observation in the tumors.
a. Cells giving rise to the breast cancer also lost expression of one copy of the p53 gene.
b. The normal BRCA1 copy is methylated, thus is not expressed.
d. The mutant BRCA1 copy is methylated