Proteome-scale tagging and functional screening in mammalian cells by ORFtag

Determining protein function in a systematic manner is a key goal of modern biology, but remains challenging with current approaches. Here, we present ORFtag, a versatile, cost-effective and highly efficient method for the massively-parallel tagging and functional interrogation of proteins at proteome scale. Using mouse embryonic stem cells, we showcase ORFtag’s utility through screens for transcriptional activators, repressors and post-transcriptional regulators. Each screen finds known and novel regulators, including long ORFs not accessible to other methods, revealing that Zfp574 is a highly selective transcriptional activator and that oncogenic fusions frequently function as transactivators.