NEGATIVE EXPANSION OF THE MYOTONIC-DYSTROPHY UNSTABLE SEQUENCE

We have analyzed the unstable fragment of the myotonic dystrophy (DM) gene in a pregnancy at 50% risk for DM. The affected father in this fa mily had a 3.0-kb expansion of the DM unstable region. The fetus inher ited the mutated gene, but with an expansion of 0.5 kb. This case repr esented a counseling problem in light of the absence of data concernin g ''negative expansion.'' Analysis of the DM gene in 17 families with 72 affected individuals revealed four more cases of negative expansion s, all of them in paternal transmissions. The possible significance of this finding is discussed.