HQL-79, a potent, selective and orally active human hematopoietic prostaglandin D synthase (H-PGDS) inhibitor, highly selectively inhibits the synthesis of PGD2, and acts as an anti-allergic agent, with a Kd of 0.8 μM and an IC50 of 6 μM. Shows no obvious effect on COX-1, COX-2, m-PGES, or L-PGDS.