In the 2022 ACR/EULAR classification criteria, MPO-ANCA positivity has become a major determinant in the classification of MPA, being assigned a score of 6, which is sufficient on its own to classify a patient with biopsy-proven small- to medium-sized vasculitis as MPA, even in the absence of other relevant features [16]. The gene discussed is MPO; the disease is microscopic polyangiitis.