Moreover, APRIL, mainly secreted by myeloid cells and osteoclasts, induces proliferation and survival of Tregs in MM [167], and the increased activity found in MM patients of indoleamine 2,3-dioxygenase (IDO), an inducible enzyme that catalyzes tryptophan to kynurenine, led to inhibition of effector T cell function and induction of Treg differentiation [194]. Here, IDO2 is linked to Miyoshi myopathy.