This is a custom reference database compiled by ORNL researchers, containing mercury methylation genes (hgcA and hgcB) from predicted and known mercury methylating microorganisms. The database contains 301 hgcA and 239 hgcB sequences from pure culture and environmental genomes publicly available from the National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov). The reference packages can be used to assign taxonomic classification to HgcA sequences from cloning, amplicon, or meta-omic datasets.