Exploratory factor analysis identified 3 underlying processes contributing to the morphologic patterns of ET. Shown for each factor are the 5 variables that contribute most heavily to each factor, with the y-axis representing their relative weight (contribution to the factor score). Factor 1 emphasises morphologic features that describe megakaryocyte clusters. Factor 2 captures the process of marrow fibrosis, weighting reticulin, new bone formation, and pyknotic megakaryocytes. Factor 3, in picking out the cellularity criteria, correlates significantly with JAK2 V617F status (P < .001).