In vivo 5F 203 efficacy was observed against MKN‐45 and AGS xenografts, and selective induction of CYP1A1, CYP2W1 and γH2AX protein expression, detected in MKN‐45, but not 5F 203‐insensitive BGC‐823 tumours excised 24 hours post‐treatment of mice (5 mg/kg 5F 203) indicate AhR signal cascade activation in vivo. This evidence concerns the gene CYP1A1 and neoplasm.