The present invention covers an ATR kinase inhibitor, particularly 2-[(3R)-3-methylmorpholin-4-yl]-4-(1-methyl-1H-pyrazol-5-yl)-8-(H-pyrazol-5-yl)-1,7-naphthyridine (“Compound A”), for use in the treatment of a hyper-proliferative disease in a subject, wherein the hyper-proliferative disease or the subject is or has been characterized by one or more biomarker(s), wherein the biomarker(s) comprise(s) a) one or more functional mutations in one or more gene(s)/protein(s) selected from RBBP8, APOBEC3A, APOBEC3B, CLSPN, ERCC, HUS1, MAD2L2, PGBD5, POLD1, RAD1, TIMELESS and/or TIPIN; and/or b) the expression of a fusion gene encoding a fusion protein selected from EWSR-ERG, EWSR1-FLI1, SS18-SSX and/or SS18-SSX2 gene/protein.