MBNL2 and Myotonia: When a KO mouse is generated for Mbnl1 in which the Mbnl2 function is also reduced (Mbnl1−/−; Mbnl2+/−), it was found that these mice are viable but develop cardinal aspects of the disease, including reduced life span, cardiac conduction blockage, severe myotonia, atrophic fibers, and progressive skeletal muscle weakness [21].