Detection of weak D types by PCR-RFLP. Four weak D types harbored point mutations that obliterated restriction sites: weak D type 1 lacks an Alw44 I site (A); weak D type 3, a SacI site (C); weak D type 5, an Alu I site (D); and weak D type 6, a Msp I site (E). In a fifth weak D type, a point mutation introduced a restriction site: weak D type 2 gained an Alu I site (B). On the left side of the gels, 100 bp ladders are shown; the position of the 500 bp and 100 bp fragments are indicated on the right side of the panels. For the PCR reaction of (A), the largest restriction fragment approximation 3,000 bp is not shown.