In vitro, BAY 1895344 is shown to be a very potent and highly selective ATR inhibitor (IC50 = 7 nM), which potently inhibits proliferation of a broad spectrum of human tumor cell lines (median IC50 = 78 nM). In cellular mechanistic assays BAY 1895344 potently inhibits hydroxyurea-induced H2AX phosphorylation (IC50 = 36 nM).