SIX5 and myotonic dystrophy type 1: However, very recently a quadruple mutant mouse was generated in which the expression of SIX5, DMWD, DMPK, and MBNL1 was reduced, and these mice recapitulate many important manifestations in congenital DM1, suggesting that changes in gene expression in genes at the DMPK locus may modulate disease severity.65