Appl Microbiol Biotechnol (2012) 93:1005–1020 codon-optimized and synthesized before being expressed in the target host. Key in overcoming expression bottlenecks is certainly a more intricate understanding of the complex aspects involved in expression of foreign genes (Fig. 3). Thus, identification of crucial hurdles involved in the inability to express genes of any metagenome should be considered as the spearhead that allows us to move forward. Ultimately, a more directed approach in improving existing gene expression systems should be envisaged. However, when metagenomic strategies are designed to yield more optimal functional screenings, there might be a risk of being trapped in an overdesigned experimental setup, which leaves insufficient room for the discovery of the “real” instead of the “similar” unknown biosphere. This holds especially true when selecting target genes from sequenced metagenomic libraries. Following this, caution should be taken when weighing the advantages and disadvantages of different DNA processing strategies for library construction. To conclude, we posit that “the great screen anomaly” is a current reality; however, the term “great expression inability” might more appropriately describe current obstacles that ham- per greater screening efficiencies. To what extent this anomaly will persist (as its predecessor “the great plate count anomaly” has) remains uncertain. Considerable fine-tuning of methods is clearly still needed to make functional screening representative of the environmental diversity and to boost its efficiency in assigning genes to function. Nevertheless, considering the pace at which innovative technologies evolve in this area, “the great screen anomaly” probably awaits a very unsure future with respect to its existence in the next decennium. Acknowledgments This work was funded by the EU Metaexplore project (KBBE-222625). 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