Principle of positional cloning approach and review the investigation of a paternally imprinted QTL (Quantitative Trait Loci) mapped and fine mapped into the IGF2 region of SSC2 with strong effect on muscle mass and fat deposition in pigs. This book aims to further pinpoint the QTN (Quantitative Trait Nucleotide). Porcine sequence encompassing the IGF2 and H19 genes was generated and annotated. The Art of Genetics and Functional Assays in Genomics Era lead to our investigation of the causative mutation for the paternally imprinted QTL with major effect on muscle mass and fat deposition in pigs. Discussion on our findings – the first QTN identification and applications.