The present invention relates to a method of identifying candidate genes and the proteins encoded by them that are useful in the inducement of hybrid vigour, hybrid debility and/or the diagnosis, prognosis and treatment of disease. In particular, the present invention relates to a method for identifying candidate genes capable of producing hybrid vigour or hybrid debility in an animal or plant, comprising the steps of: (i) comparing the mRNA sequence of alleles of candidate genes isolated from an animal or plant which exhibits hybrid vigour or hybrid debility with the nucleotide sequences from the corresponding alleles isolated from the parents of said animal or plant; (ii) identifying mRNA sequence differences in the alleles from said animal or plant which exhibits hybrid vigour or hybrid debility which codes for amino acid sequence variation; and (iii) identifying that the amino acid sequence variation between alleles of the candidate gene in said animal or plant is encoded by mRNA sequences which are located within two or more different exons within the candidate gene.