Three cases of ring chromosome 2, one derived from a paternal 2/6 translocation.

Two cases of primary ring chromosome 2 and one case of a ring secondary to a paternal 2/6 translocation are described and compared with a fourth case of ring 2 from the literature. The breakpoints in two cases are identical and the same as the breakpoint on chromosome 2 in the composite 2/6 ring. The three primary rings have a number of symptoms in common, but other cases are needed to identify a possible deletion syndrome. The levels of the enzyme MDH-1 were within the normal range in Cases 1 and 2. Thus we localised the locus for MDH-1 to segment 2p23 leads to p25; this had previously been localised to segment p23 leads to pter.