AbstractSkeletal muscle fibre types are determined by the activity of motor neurons, hormones, stretch and probably cell lineage. The electrical activity of the motor neurons might, through an unknown pathway, influence the expression of a unique set of muscle specific genes in the different fibre types. MRF4 is a member of a family of myogenic basic helix-loop-helix transcription factors (bHLH) that is important for the development of muscle fibres. These transcription factors are also suggested to be important in adult muscle, where they might be a possible link between electrical activity and expression of muscle specific genes. The myogenic bHLH transcription factors MyoD and myogenin has been shown to possible be involved in the regulation of fibre type specificity. This role is also proposed for MRF4, based on the high amount of MRF4 mRNA that is expressed in all adult skeletal muscle types and that it seems to be preferentially expressed in slow fibre types.In this thesis I wanted to further examine if MRF4 might possible contribute in the regulation of fibre type specificity in normal adult skeletal muscles. MRF4 was overexpressed in the fast glycolytic calf muscle; extensor digitorum longus (EDL). Plasmids containing the MRF4 gene and a reporter gene were transfected into the muscle by electroporation. After 14 days there was no change in the cross-sectional area or SDH activity in MRF4-transfected fibres compared to the control (sham)-transfected fibres. There were a significant higher number of 2b fibres in both MRF4- and sham-transfected fibres compared to normal, non-transfected fibres. This higher amount of 2b fibres is speculated to be caused by a possible selective transfection of the largest fibres as an effect of electroporation properties, or an effect of the lacZ itself or both. These results indicate that MRF4 induces no change in MyHC isoforms, cross-sectional area and oxidative metabolism in normal adult skeletal muscle and this does not support the hypothesis that MRF4 may have a role in regulating fibre type specific patterns of muscle gene expression in adult skeletal muscle.