- A paternally expressed QTL affecting skeletal and cardiac muscle mass in pigs maps to the IGF2 locus. Jeon, J.T., Carlborg, O., Törnsten, A., Giuffra, E., Amarger, V., Chardon, P., Andersson-Eklund, L., Andersson, K., Hansson, I., Lundström, K., Andersson, L. Nat. Genet. (1999)