1-(2-Thiazolylazo)-2-naphthol is a high affinity Rio Kinase 2 (RIOK2) inhibitor (Kd = 200 nM). Exhibits >6-fold selectivity for RIOK2 over a panel of 456 kinases. Inhibits ERG protein and potently inhibits growth of ERG-positive VCaP prostate cancer cells in vitro (IC50 = 169 nM) but not ERG-negative cell lines. Also inhibits growth of VCaP xenografts in mice.