4-Hydrazinobenzoic acid (p-Hydrazinobenzoic acid, HBA) has been reported as an ingredient of the cultivated mushroom Agaricus bisporus. It is carcinogenic to rodents. Mechanism of DNA damage by 4-hydrazinobenzoic acid has been studied by using 32P-labeled DNA fragments obtained from the human p53 and p16 tumor suppressor genes. It may be used for the preparation of 4-(3-(2-(tert-butoxycarbonylamino)ethyl)-5-hydroxy-1H-pyrazol-1-yl)benzoic acid.