Candidate gene studies also have been valuable in looking for consistency across species in the impact of certain genes or gene variants. Invertebrate models (e.g., the fruit fly Drosophila or the worm Caenorhabditis elegans) offer much more powerful tools to manipulate the genome than do rodents (Lovinger and Crabbe 2005). However, to be able to use such models, researchers first need to demonstrate that corresponding genes exist in these organisms and that they actually have similar functions. One example of such convergence of evidence is the finding that a small signaling molecule called neuropeptide Y (NPY) and its receptors play a role in alcohol intoxication in mice, rats, and Drosophila (Chen et al. 2008b; Gilpin et al. 2004; Thiele et al. 2002). A meta-analysis of human association data, in contrast, found no clear evidence that polymorphisms in the gene encoding a precursor of NPY are associated with alcohol dependence (Zhu et al. 2003). However, some genes encoding NPY receptors may play a role in alcohol dependence and withdrawal (Wetherill et al. 2008). Finally, certain signaling proteins (e.g., epidermal growth factor
