I don't know if this is the same thing as what you are referring to, but when we went in to get our DNA done to locate the CAH gene, they couldn't find my husband's CAH gene. The reason for this (they told us) is that they only test for the top 5 - 7 most common strains of CAH. If they don't find a match there, they may continue on with the last few strains until they find a match or determine it to be a mutation. I guess you could say that they were undetectable in the first round of testing. I wish you good luck.