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He is referring to the fact that all Agaricus mushrooms are known to contain agaritine, a known carcinogen and hydrazine derivative, in varying amounts.
Agaritine is non volatile and not destroyed in cooking (though this does reduce the amount somewhat), and as a result many people suggest eating Agaricus species only occasionally.
The truth is that many Agaricus species also contain anti-cancer substances, and agaritine might not be present in sufficient amounts to cause an increased risk of cancer. (read GGMM, the section on A. bisporus, for details and refs.)
Since button mushrooms are, in my opinion, the least tasty ones out there, I avoid them, only eating the occasional portobella and blazei, when they are available.
If you are worried about cancer and fungi, research aflatoxins, which are a VERY common contaminant of peanuts (and peanut butter), and are among the most carcinogenic compounds known. Only peanuts grown in dry regions (Calfornia, for example) are considered free of aflatoxin, and a number of health-food brands specify growing conditions for this reason.