In conclusion, but further work is required to elucidate the nature of the subtle changes and thus render them suitable for use as a transfusion therapy. Certain membrane and cytoskeleton proteins are essential for the structure and function of RBCs. Many of the key membrane proteins associate to form two multi-protein complexes, the band 3-ankyrin macrocomplex17, and the 4.1R junctional complex, linking the red cell membrane to the underlying cytoskeleton. The latter complex proposed predominantly from studies in the mouse. Importantly, all proteins in these complexes, except the Duffy blood group, were detected in our day 21 C19 Regorafenib citations erythroid cell proteome analysis mass spec data. Duffy is a large, glycosylated protein with few trypsin sites resulting in a large peptide that is not compatible with mass spec identification. Duffy expression in the C19 erythroid cells was therefore confirmed by western blot. Other notable RBC membrane proteins expressed by the C19 erythroid cells were aquaporin 1 and CD44. CD44 was reported in the mass spectrometry data, but there was no unique peptide. The aim of our study was to determine how similar erythroid cells differentiated from iPSC are to adult erythroid cells generated in vitro, and whether there are any gross differences which could impact on their potential for use as therapeutic source of red cells. Reassuringly, our initial proteome analysis of orthochromatic erythroid cells differentiated from C19 iPSCs revealed no major differences regarding expression of hallmark RBC proteins, particularly all known components of the 4.1R and ankyrin membrane complexes. Of these.30 were identified in the proteome data from earlier erythroid cells, the level of which was consistent between all three iPSC and adult erythroid cells. Further interrogation of the comparative proteome data showed that the level of 50 proteins involved in RNA processing was also consistent between the iPSC and adult erythroid cells. There was however differential expression of some of the 1989 quantified proteins between the iPSC and adult cells, 11% and 1.9% for a 2 fold and 5 fold threshold respectively. These values were similar to those between CB and iPSC erythroid cells, 9.3% and 1.5% respectively. Some of the differences will be due to intrinsic variability between individuals. However, our data suggest that the proteome of the iPSC erythroid cells is more aligned to that of CB cells. An obvious feature of fetal erythroid cells is the expression of c-globin. The iPSC erythroid cells expressed c- but little b-globin, likely due, at least in part, to the low level of KLF1 and absence of BCL11A in these cells, both of which are known to be required for the developmental switch from fetal to adult globin expression. However, the iPSCderived erythroid cells expressed a higher level of embryonic globins than CB cells. We therefore investigated the expression of proteins reported to be unique to embryonic erythroid cells to see if the iPSC-derived cells exhibited a more embryonic phenotype.