This transgene consists of the full-length adult mouse (BALB/c) ventricular isoform of Myl2 (regulatory myosin light chain 2) with serines 14, 15, and 19 each mutated and replaced by alanine, under the control of the murine alpha-myosin heavy chain promoter. Replacing the three serines with alanines results in a nonphosphorylatable Myl2. Examination of atrial myofibrillar protein composition shows a progressive replacement of the atrial isoform with the mutated ventricular form as mice age.