The third type, unmatched metagenomic protein databases, is built from metagenomic data from the same overall ecosystem rather than the specific sample. Examples include the Integrated Gene Catalog (IGC) of the human gut microbiome , the (expanded) human oral microbiome database (HOMD) , and the unified Global Ocean Microbiome Catalog (GOMC) . These databases, commonly used for well-characterized ecosystems, are more accessible and enable cross-study comparisons. However, they are less specific, and (much) larger in size than matched metagenome or restricted reference databases, often resulting in lower identification rates and possibly inflated FDR due to increased database size and complexity .