Examination of peripheral blood and bone marrow biopsy specimens, as well as ancillary studies for both VEXAS patients. (A-B) There were no cytoplasmic vacuoles or overt dysplastic changes in neutrophils (A) or monocytes (B) in peripheral blood (representative pictures from patient 1) Original magnification ×1000. (C-E) Markedly hypercellular bone marrow (C) showed extensive granulocytic hyperplasia with complete maturation (D). (E) This effect is also illustrated by MPO immunostaining in a representative image from patient 2. Original magnifications ×200 (C), ×400 (D-E). (F) By immunohistochemistry, there was no significant increase in CD34⁺ blasts (3% to 4% of bone marrow elements). Original magnification ×400, a representative image from patient 2. (G-H) Characteristic vacuoles were found in erythroid precursors (pronormoblasts) (G), and profound megaloblastic changes were seen (H) in patient 1, although overt nuclear membrane irregularity was not appreciated. Original magnification ×1000. (I-K) Prominent vacuoles were also identified in myeloblasts (I), promyelocytes (J), and immature monocytes (K) in patient 2. Original magnification ×1000. (L) Frequent immature-appearing megakaryocytes showed an increased nuclear/cytoplasmic ratio, whereas typical dysplastic changes were not identified in patient 2. Original magnification ×1000. (M) By flow cytometry, granulocytes exhibited an abnormal maturation pattern illustrated by plots with CD13 vs CD11b in patient 2. (M) Dashed arrow indicates the normal granulocytic maturation pathways. (N) UBA1 mutations p.M41L and p.M41V were confirmed by Sanger sequencing in patients 1 and 2, respectively. Top codons, patients’ variants; bottom codons, reference codons. M, mutant; W, wild-type.