MBNL1 and myotonic dystrophy type 1: To test if CUG-RNA aggregates aberrantly sequester the alternative splice factor, Mbnl1, we stained both α-MHC-LacZ and α-MHC-LacZ-(CTG)400 cardiomyocytes and normal human and DM1 myoblasts with anti-MBNL1 (MB1a) monoclonal antibodies [31].