Mice were treated as described in Fig. 3a. Blood and spleen were analysed 16 weeks after transplantation of Lin– mouse HSPCs containing human HBB alleles. a, Representative images of blood smears. One blood smear image was collected per mouse. Scale bars, 25 μm. b, Representative phase contrast images of peripheral blood incubated for 8 h with 2% oxygen. Nine images of more than 50 cells per image were collected per sample. Scale bars, 50 μm. c, Quantification of sickled cells. More than 300 randomly selected cells per condition were counted by a blinded observer. d, Mass of dissected spleens. e, Histological sections of spleens of recipient mice 16 weeks after transplantation. Splenic pathologies in mice that received unedited donor HBBS/S HSCs include excessive extramedullary erythropoiesis and vascular congestion indicated by RBC pooling (bright red colour) resulting in expansion of red pulp (RP), reduction in white pulp (WP), and splenomegaly. Images were taken at 10× magnification and were processed, stained and photographed at the same time under identical conditions. Three images of each spleen were collected from different parts of the organ for each mouse. Scale bars, 100 μm. Unedited HBBS/S, n = 6 mice; edited HBBS/S, n = 6 mice; HBBA/S, n = 2 mice. Data shown as mean ± s.d., with individual values as dots. Statistical significance was assessed using one-way ANOVA with Šidák’s multiple comparisons test of the edited HBBS/S values compared to each other group to calculate P values.