Anti-Er alloantibodies are specific for antigenic sites on Piezo1. Flow cytometry histograms illustrate cell surface labeling of indicated wild-type or mutant Piezo1 constructs overexpressed in an endogenous Piezo1 knockout (KO) BEL-A cell line using antibodies eluted from plasma of individuals (P3, row A; P11, row B; P12, row C; and P13, row D) with mutations in Piezo1 as labeled. BEL-A Piezo1 KO cells and endogenous Piezo1 are shown in column 1, with overexpressed wild-type Piezo1 in column 2. Columns 3 to 7 show overexpressed mutant Piezo1 constructs as labeled. Results are summarized in the grid below histograms. O-E WT, overexpressed wild-type; +, positive; −, negative; (+), weakly positive.