Because certain haplotypes are inherited among siblings more frequently than expected merely by chance, there is a 25-30 percent probability of finding a 6:6 match within a family. If one antigen is mismatched, it is a 5:6 match, which clinicians will accept because it increases the odds of finding a donor. When doctors accept a 4:6 match, the likelihood of finding a donor increases even more.

As the number of HLA antigen matches decreases among donors and recipients, the chances of finding a donor increase:

Antigen match Percent chance of finding a donor:
6-antigen match 25-30%
5-antigen match 40-50%
4-antigen match 50-60%
3-antigen match >90%