A significantly decreased fate decision of MEP towards the erythroid lineage and an early block in definitive erythropoiesis results in severe anaemia of conditional Ak2‐knockout embryos and likely leads to embryonic lethality of these mice between E13.5 and E18.5, similar to erythropoietin receptor‐deficient mice [33, 47]. The gene discussed is AK2; the disease is anemia (phenotype).