TYK2 and rheumatoid arthritis: Cumulative evidence suggests that the JAK-STAT pathway is involved in bone metabolism.[17] JAK family proteins consisting of JAK1, JAK2, JAK3, and tyrosine kinase 2 induce production of pro-inflammatory cytokines through activation of STAT proteins.[29] Some researchers investigated whether the JAK signal pathway plays a crucial role in osteoclastogenesis in RA-like inflammatory arthritis.