The mOTUs tool uses phylogenetic marker gene (MG)-based operational taxonomic units (mOTUs) to profile microorganisms at species-level resolution when using metagenomic data as input. Since the MGs represent housekeeping genes, mOTUs can also be used to profile transcriptionally active community members when using metatranscriptomic data as input. Furthermore, MG-based SNV profiling provides an efficient alternative to using whole genome sequences to compare microbial strain populations.