Abstract
Metagenome-assembled genomes (MAGs) are microbial genomes reconstructed from metagenome data. In the last few years, many thousands of MAGs have been reported in the literature, for a variety of environments and host-associated microbiota, including humans. MAGs have helped us better understand microbial populations and their interactions with the environment where they live; moreover most MAGs belong to novel species, therefore helping to decrease the so-called microbial dark matter. However, questions about the biological reality of MAGs have not, in general, been properly addressed. In this review, I define the notions of hypothetical MAGs and conserved hypothetical MAGs. These notions should help with the understanding of the biological reality of MAGs, their worldwide occurrence, and the efforts to improve MAG recovery processes.