Men currently account for about one third of the total number of donors in the Tobias Registry. The amount of blood stem cells that a person can donate, and that a patient needs, depends on body volume. The donor needs to be as big or bigger than the recipient, and men tend to have a higher body weight than women. Half of the patients are male and thus generally require a larger amount of stem cells than the corresponding female patient. Many young women are periodically unavailable as donors due to pregnancy and breastfeeding. This means that around 10% of the women surveyed in the registry are unable to become donors. That is why we would like to register more young men, as there is a general shortage worldwide.