A phase detector for a clock and data recovery circuit from random non-return-to zero data signal includes a plurality edge-triggered flip-flops. The at least two segregated pools are stacked. In addition, this invention relates to methods identifying compounds that bind to PB2 polypeptide fragments comprising the RNA cap binding pocket, preferably inhibit the interaction with RNA caps or analogs thereof, by using said PB2 polypeptide fragments, preferably in a high-throughput setting.