Osteopetrosis is occasionally observed in patients with XL-NEMO deficiency (OL-EDA-ID, osteopetrosis, lymphedema, hypohidrotic ectodermal dysplasia, and immunodeficiency), but not in patients with inborn errors of IKK-α, NIK, RelB, or NF-κB2. Here, RELB is linked to osteopetrosis.