*3.3.1. RFLP (Restriction Fragment Length Polymorphism)*

Restriction Fragment Length Polymorphism (RFLP) methods rely on the ability of certain enzymes to recognize exact DNA nucleotide sequences and to cut the DNA at each of these points [14]. Thus, the frequency of a particular sequence will determine the lengths of DNA produced by cutting with a particular enzyme.

The DNA for one HLA (Class II) antigen, e.g. DR15, will have these particular enzyme cutting sites (or "restriction sites") at different positions compared to another antigen, e.g. DR17. Consequently, the lengths of DNA observed when DR15 is cut by a particular enzyme, are characteristic of DR15 and different to the sizes of the fragments seen when DR17 is cut by the same enzyme [15].
