Verastegui et al. combined stable-isotope probing (SIP) with functional metagenomics to identify new glycoside hydrolases produced by soil microorganisms, which have multiple biotechnological applications, owing to their ability to degrade complex carbohydrates. The authors added 13C-labelled carbohydrates to different soil samples and enriched isotope-labelled (heavy) DNA by centrifugation. Sequencing of the heavy DNA revealed several taxa that could degrade the labelled complex carbohydrates, and annotation of the metagenomic data revealed several new potential glycoside hydrolase genes. The authors then constructed a cosmid library using the heavy DNA to carry out functional screens and identified several clones that can degrade cellulose and other complex plant polymers. These data show how SIP and metagenomics can be combined to identify novel functional genes from environmental samples.