Possible selective pressures promoting adjacency include retention within clusters sharing common regulatory elements and/or retention as a means of promoting the co-inheritance of co-adapted sets of alleles among these genes. Gene duplications offer evolutionary opportunities for creation and/or specialization of gene function. One of the duplicates is now free to acquire new molecular specificity by Epoxomicin altering its binding properties, change its response to regulatory signals that can alter tissue specificity of expression, mutate its transcript splicing patterns to produce distinctly new molecular forms of protein, or even divide its molecular functions between the two duplicates by having each partner lose complementary functional domains. Duplicated genes within clusters also provide a unique environment for genomic phenomena that may not be found among dispersed duplicates. In addition to the possibility of shared regulatory elements, there is the potential co-inheritance of allelic combinations, which in the extreme case can lead to near total linkage disequilibrium of alleles within clusters, the expansion and contraction of tandem gene copy number, and gene conversion events which can act to either homogenize or diversify the DNA sequences of clustered genes. Whatever the order and types of genomic events that have occurred, and whatever evolutionary forces have been operative, it is clear that together they have left contemporary genomes with extensive genomic clustering of paralogous genes. In the human genome, for example, we find that greater than one in five genes are organized in this way, making clustering an important feature of contemporary genomic organization and providing an impetus to understand the features of these arrangements, how they might affect genomic function, and what role if any they play in the survival of species. As a means of describing some of the general characteristics of paraclusters and providing an evolutionary perspective, we have integrated the information from multiple datasets to create catalogs of the paraclusters present in a diverse set of vertebrates, two invertebrates, Drosophila melanogaster and Caenorhabditis elegans, the higher plant Arabidopsis thaliana and the yeast Saccharomyces cerevisiae. Overall genome Afatinib statistics for each of the species tested are listed in Table S1. Two additional datasets place their emphasis on the presence of shared functional domains, relying on Hidden Markov Models for representing structural features. Here we have imputed paralogy when two proteins share domains and are located in close proximity beyond chance expectation, which depends on the frequency of the domains across the entire genome. The SCOP superfamilies dataset uses domain classification to assert common evolutionary origin between proteins even with low sequence similarity.