Accumulating evidence supports the polygenic nature of most complex diseases, suggesting the involvement of many susceptibility genes with small effect sizes. Although hundreds of genes may underlie the genetic architecture of complex diseases, those involved in a given disease are probably not randomly distributed, but likely to be functionally related. Protein-protein interaction networks have been used to evaluate the functional relatedness of susceptibility genes. However, these networks do not account for tissue specificity, are limited to protein-coding genes, and are typically biased by incomplete biological knowledge. Here, we present Gene Link Inspector Through Tissue-specific coExpRession (GLITTER), a web-based application for assessing the functional relatedness of susceptibility genes, either coding or noncoding, according to tissue-specific gene expression profiles. GLITTER can also shed light on the specific tissues in which susceptibility genes might exert their functions. We further demonstrate examples of how GLITTER can evaluate the functional relatedness of susceptibility genes underlying schizophrenia and breast cancer, and provide clues about etiology.