YBR1012 (YBR136w) is an essential gene from Saccharomyces cerevisiae identified during the systematic sequencing of part of the right arm of chromosome II. We previously constructed a conditional allele of YBR1012 based on antisense RNA, by inserting a small fragment of this gene downstream from the inducible UASGAL10-CYC1 promoter. Several other antisense RNA constructions have since been made and their activity tested. The response of the system appears to be very delicate, as the presence or absence of 13 nucleotides of polylinker in the 300 nucleotide antisense transcript can dramatically modify its effectiveness. The most effective antisense RNA construction was used in flow cytometry studies to investigate the role of ybr1012p. The results show that during the antisense RNA block some 80% of the cells are arrested with their DNA unreplicated, suggesting that Ybr1012p is needed for progression through G1 or early S phase.