What are Null Alleles and Common and Well Defined (CWD) Null Alleles?

Some irregularities in DNA sequences may result in no expression or expression of totally non-functional protein. Those irregularities could be missing base(s) (deletions), have extra base(s) (insertions), or have base substitutions that result in premature termination of HLA protein synthesis (early stop codons). In certain cases those irregularities could happen in regions of the genes that are outside of the ARS regions. Therefore by just testing for ARS DNA sequences, one may miss those null alleles. This could result in calling a mismatch typing as a match and could affect the success of the transplant outcome.

Those null alleles that have seen at least five times in different ethnic populations are tested in donors and patients. We call this extra test as testing for CWD null alleles. (3x High resolution).