EPX and hereditary elliptocytosis: Tissue HE is defined by one or more of the following criteria: (i) eosinophils constitute more than 20% of all nucleated cells in bone marrow sections, (ii) a pathologist identifies extensive (massive) eosinophil infiltration in tissue beyond normal physiological ranges, or (iii) immunostaining reveals significant extracellular deposition of eosinophil granule proteins, such as eosinophil major basic protein (eMBP1) or eosinophil peroxidase (EPX) [1▪,3▪,4].