Bacteriophage T4 AsiA, a strong inhibitor of bacterial RNA polymerase, was the first antisigma protein to be discovered. Recent advances that made it possible to purify large amounts of this highly toxic protein led to an increased understanding of AsiA function and structure. In this review, we discuss how the small 10-KDa AsiA protein plays a key role in T4 development through its ability to both inhibit and activate bacterial RNA polymerase transcription. (c) 2004 Elsevier Inc. All rights reserved.