The effects of immunosuppressive drugs on BK polyomavirus (BKV) replication have been compared in a new study. Sirolimus inhibited viral replication up to 24 h postinfection, but not thereafter, and similar results were found for other mTOR inhibitors and ciclosporin A. Conversely, tacrolimus — a calcineurin inhibitor — activated BKV replication. The effects of tacrolimus and sirolimus were mediated through FKBP12 in renal tubular epithelial cells. The researchers propose that these data provide a rationale for clinical trials aimed at reducing the risk of BKV replication in renal transplantation.