IDO1, the rate-limiting enzyme in the kynurenine pathway, the excessive activation of the KYN pathway can diminish tyrosinase activity in vitiligo-affected areas, inhibit tyrosinase expression in MCs and KCs co-cultures, and reduce melanosome numbers in the 3D human skin reconstruct model (Ferreira Branquinho et al., 2022). The gene discussed is TYR; the disease is vitiligo.