Environmental gene tags (EGTs) are short DNA sequences used to characterize and distinguish microbial environments. EGTs include fragments or complete sequences of genes that are important to survive in specific environmental conditions and hence are present in the genomes of many microbes of one environment and less frequent or even completely absent in another environment. The frequencies of all EGT sequences in the whole genomic content of a specific environment is used as a fingerprint that characterizes the environmental adaptation of microbes.
Genes that are more frequently found in one type of microbial community compared to another are supposed to provide beneficial functions for the community living in a specific environment.
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Metagenomics: DNA sequencing of environmental samples
Susannah Green Tringe and Edward M. Rubin, Nature Reviews Genetics