Ritlecitinib (1-[(2S,5R)-2-methyl-5-(7H-pyrrolo[2,3-d]pyrimidin-4-ylamino)piperidin-1-yl]prop-2-en-1-one; WHO ACT code L04A-F08; CAS No: 1792180-81-4; PF-06651600) is an irreversible inhibitor of JAK3 and TYK2 that Pfizer is developing under the trade name LITFULOTM for treatment of alopecia areata (Figure 1). The gene discussed is JAK3; the disease is alopecia areata.