Furthermore, HD and DM1 mice were crossed with mice haploinsufficient for Fen1. Somatic CAG/CTG instability was unchanged in HD and DM1 mice heterozygous for Fen1, regardless of the tissues analyzed, though a modest effect on germline instability was observed in HD mice haploinsufficient for Fen1 [51,52]. The gene discussed is FEN1; the disease is myotonic dystrophy type 1.