In metagenomic sequencing, the DNA sequences of multiple organisms are analyzed as a single data set. This approach is used to characterize com- plex microbial communities, such as those in marine environments, where in- dividual species—including many not yet identified—cannot be cultured and sequenced one by one. Metagenomic sequence data reveal the overall gene number and an estimate of the collective metabolic capabilities of the com- munity. Over 3 million genes have been identified in a metagenomic analysis of the microorganisms that inhabit the human gut, representing some 1000 bacterial species. While most humans share a common core set of about 60 gut microorganisms, significant differences appear to correlate with metabolic variables such as body mass.