et al., 2003; Spence et al., 2009) as well as preproNPY (c.f., Koob and Le Moal, 2006) on chromosome 4 for the P vs. NP line-pair and CREB binding protein (Crebbp) and a MAP kinase (Mapk8ip3) on chromosome 10 (Bice et al., 2010) as well as proenkephalin on chromosome 5 and neuronal nitric oxide synthase-1 on chromosome 12 (c.f., Koob and Le Moal, 2006) for the HAD vs. LAD line-pairs.