Before revision:  While several studies have shown that strong selection pressures enrich epistatic interactions in an evolving protein, little is known about how epistatic interactions themselves change over time when selection pressures are not constant. Obtaining fitness data to measure epistasis in a protein over significant and evolutionarily relevant time scales is a daunting task, rendering a study of the long-term evolution of epistasis in a proteincurrently infeasible .
After revision:  While several studies have shown that strong selection pressures enrich epistatic interactions in an evolving protein, little is known about how the epistatic interactions change under selection in the long-term evolution of epistasis in a proteincurrently infeasible .