CYP26A1 and rheumatoid arthritis: The authors found that limiting RA levels by CYP26‐expressing cells in the BM niche favours HSC proliferation.85 These CYP26‐expressing stromal cells may play an important role in providing an acute promyelocytic leukaemia‐permissive BM environment.86 The exact role of RA and the control of its local concentration on immune cell differentiation remain to be further explored by dissecting the different niche compartments for granulopoiesis and HSC maintenance.