Micro-array analysis (as opposed to the Exome sequencing they are starting to offer, which may be useful for BRCA diagnosis) focusses on frequently occurring single nucleotide polymorphisms (substitution of one base for another). The majority of these are benign.
By looking at lots of commonly occurring variants it is hoped that they can map regions of the genome that may harbour other mutations that may actually have an effect.
Baynes et al., 2007 [1] showed that none of the commonly occurring SNP's in BRCA (> 1-5% of the population) have a significant risk in cancer.
I stand corrected. It looks like 23andme is testing for just a few high-penetrance but uncommon mutations that are mostly only of interest to Ashkenazi Jews.
Myriad, the company that owns the BRCA patents, has been the only player in town for almost 20 years.
There is currently a case before the Supreme Court trying to invalidate Myriad's patents, as well as all patents on human DNA.
The issue, however, is that Myriad's value is no longer in their patents, but in the data they collected over the past 20 years.
Myriad charges ~$3k for their tests, while a full sequence of your entire genome is only ~$5k, and will tell you the same genetic data. But Myriad has been able to correlate much more of your DNA and the BRCA regions with cancer risk, and that data is not public.
