Do bacterial cryptic genes really exist?

Cryptic genes have been defined as phenotypically silent DNA sequences, usually not expressed during the life cycle of a microorganism, but capable of expression in a few members of a large population by mutation, recombination, insertion processes, or other genetic mechanisms. Recently, the crypticity of several genetic systems has been questioned. It appears that in many cases cryptic genes are silent only under the experimental conditions analysed and that their expression can be induced in the natural environment. Therefore, we propose that cryptic genes might not be a peculiar class of uniquely regulated genes, but rather genes encoding unusual functions. (C) 2000 Editions scientifiques et medicales Elsevier SAS.