In 2020, a 2-month-old ethnically Danish girl was diagnosed with β-thalassemia after presenting with persistent jaundice. The peripheral blood smear showed significant aniso- and poikilocytosis, increased number of reticulocytes and erythroblastosis. Trio analysis of the index patient and both parents was performed by whole-genome sequencing. Here, both parents were found normal, however the analysis revealed an apparently de novo HBB:c.444A > C variant in the child. The child has recently been discharged three months after a successful bone marrow transplantation with a matched sibling-donor.