The heme defect exhibited by Grab-deficient erythroblasts is due to reduced iron uptake. (A-B) Representative fluorescence images (A) and quantification (B) of calcein green and RPA in wild-type and Grab knockout MEL cells. N = 30 for each group; **P < .01. (C-D) Representative fluorescence images (C) and quantification (D) of calcein green and RPA in wild-type and GRAB knockout K562 cells. N = 30 for each group; **P < .01. (E-F) Quantification of iron content in differentiating MEL cells (E) and K562 cells (F) by ICP-MS. **P < .01. (G) Flow cytometric analysis showing reduced uptake of Alexa Fluor 488–labeled holo-transferrin (Tf-Alexa 488) in differentiating Grab-deficient MEL cells. *P < .05; **P < .01. (H-I) Supplementation of iron citrate and hinokitiol restored heme synthesis in Grab-deficient MEL cells (H) and K562 cells (I) following chemically induced differentiation. *P < .05; **P < .01. (J) Supplementation of iron citrate and hinokitiol significantly rescued the heme defect induced by rab3il1 depletion. Zebrafish embryos at 72 hours postfertilization were analyzed. **P < .01. DIC, differential interference contrast.