A competitive polymerase chain reaction has been developed for quantitation of BRCA1 mRNA. In human cancer cell lines, the amount of BRCA1 mRNA is relatively low, ranging from 6 to 38 copies per cell. The decay rate of these transcripts in actinomycin-treated cells indicates that the half-life of these molecules is about 4 hr, suggesting that the low concentration of BRCA1 messages is not due to molecular unstability. In human lymphoblastoid cell lines derived from patients carrying germ-line alterations of BRCA1, the amount of BRCA1 mRNA per cell is lowered only in cell lines exhibiting alterations leading to specific loss of transcripts from the mutated allele. These data indicate that the amount of BRCA1 available in these cells can be related directly to the number of "active" allele.