ERAP1 and ankylosis: Here we show that deficiency of ERAP1 was sufficient to cause a spontaneous and rapid onset of spinal ankylosis, calcification of the anterior longitudinal ligament (ALL), sacro-iliac (SI) erosions, and systemic osteoporosis in mice, making this a valuable model to use for studying the pathogenesis of these skeletal abnormalities that are also found in human AS.