Examples of paraclusters of this type include olfactory receptors, C2H2 zinc finger genes, immune system genes, protocadherins, histones, HOX clusters, cytochrome P450s, keratins, neurotransmitter receptors, kallekreins, serine peptidase inhibitors and cytokines. Most of these classes involve clustered genes having undergone lineage-specific gains and losses, a contributing factor to the diversification of gene families. Table 3 summarizes the numbers of paraclusters and their gene content for all of the species tested. Table S4 provides the breakdown of results for each dataset in each species. With some variation, the basic picture seen in humans extends across all of the species examined, both animal and plant, with the sole exception of S. cerevisiae. Multicellular organisms show similar fractions of their genes in paraclusters and similar dimensions of paraclusters. The chicken genome is the notable exception among vertebrates. The lower MK-4827 percentage of chicken genes found in paraclusters is not the consequence of incomplete annotation as only 8% of genes lack annotation in that species. Nor is it due to the presence of a large number of very small chicken chromosomes, which act to divide the genome up into small fragments. Indeed, four of the largest chicken paraclusters were found on micro chromosomes; two keratin clusters, and an immunoglobulin cluster on chromosome 27 and the Major Histocompatibility Complex on chromosome 16. And on a genome wide scale, there was no difference in the density of paraclusters in small v. large chromosomes, or indeed among all chromosomes. Rather, the relative lack of paraclusters in chickens reflects a paucity of the common, large gene family expansions that characterize mammals. Even homologous clusters that were relatively large in the chicken showed even greater expansions in human and other mammalian species; for example, an olfactory receptor cluster containing only 8 genes in the chicken is homologous to a cluster with 74 genes in human and 161 in mouse. This lack of larger paraclusters is likely related to the substantial reduction in segmental duplications and pseudogenes found in general in the chicken genome. Whether or not this is a common feature of avian species or limited to a narrower clade can only be determined when annotated genomes of other avian species become available, The evolutionary origins of paraclusters can sometimes be deduced using the InParanoid GDC-0941 database which describes gene to gene homology between any two represented species, distinguishing in-paralogs from out-paralogs. With respect to paraclusters, inparalogs are those genes that have expanded within paraclusters since the two compared species diverged from a common ancestor, whereas out-paralogs already existed within the paraclusters of the common ancestor.