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The CD33-related sialic acid binding Ig-like lectins (CD33rSiglecs) are predominantly inhibitory receptors expressed on leukocytes. They are distinguishable from conserved Siglecs, such as Sialoadhesin and MAG, by their rapid evolution. A comparison of the CD33rSiglec gene cluster in different mammalian species showed that it can be divided into subclusters, A and B. The two subclusters, inverted in relation to each other, each encode a set of CD33rSiglec genes arranged head-to-tail. Two regions of strong correspondence provided evidence for a large-scale inverse duplication, encompassing the framework CEACAM-18 (CE18) and ATPBD3 (ATB3) genes that seeded the mammalian CD33rSiglec cluster. Phylogenetic analysis was consistent with the predicted inversion. Rodents appear to have undergone wholesale loss of CD33rSiglec genes after the inverse duplication. In contrast, CD33rSiglecs expanded in primates and many are now pseudogenes with features consistent with activating receptors. In contrast to mammals, the fish CD33rSiglecs clusters show no evidence of an inverse duplication. They display greater variation in cluster size and structure than mammals. The close arrangement of other Siglecs and CD33rSiglecs in fish is consistent with a common ancestral region for Siglecs. Expansion of mammalian CD33rSiglecs appears to have followed a large inverse duplication of a smaller primordial cluster over 180 million years ago, prior to eutherian/marsupial divergence. Inverse duplications in general could potentially have a stabilizing effect in maintaining the size and structure of large gene clusters, facilitating the rapid evolution of immune gene families. [ABSTRACT FROM AUTHOR]