Integration of titin-eGFP into the sarcomere. Expression and localization of titin-eGFP were documented in the heart, quadriceps, and soleus of homozygous, adult knockin mice. Coimmunofluorescence staining with antibodies directed against the Z-disc protein α-actinin (A), titin’s Z1/Z2 epitope (B), and titin’s M-band epitope M8/M9 (C) confirmed that titin-eGFP was properly integrated into the Z-disc and M-band of the sarcomere where it colocalized with the M8/M9 epitope in all striated muscle tissues analyzed. Bar, 10 µm.