The New England Journal of Medicine recently released a paper on an extremely rare set of boy/girl sesquizygotic twins. Early in the pregnancy, it was noted on ultrasound that the twins shared a placenta, indicating they were likely identical twins. Sesquizygotic twins have been reported in the literature before, but this is reportedly the first case of it being detected during a pregnancy.
When you have an allogeneic bone marrow or stem cell transplant, the blood-producing cells in your bone marrow are killed off by radiation or chemotherapy and then replaced with functioning cells from another person. The technical term for this process is allogeneic hematopoietic cell transplantation.