I think this depends on it is the unrelated or siblings. For unrelated donor, you want the mismatched allele of the recipient is shared with the donor's mother, what we call it NIMA match. For siblings, when there is one allele mismatch, you prefer the one from the donor is NIMA for recipient rather than NIPA, because theoretically the recipient is tolerant to that NIMA (HvG permitted). And vice versa, because the donor is also supposed to be tolerant to the allele that has been given to his sibling but not him (GvH permitted).