Vertebrate Hb is a tetramer composed of two a- and two b-chains. Mb is a monomer that is mainly located in the cytoplasm of the myocytes in heart or skeletal muscles. Mb stores O2, facilitates O2 diffusion to the mitochondria, and may also be involved in the decomposition of NO. Within the past ten years, sequencing of CPI-613 expressed sequence tags and whole genomes revealed the presence of additional globin types in vertebrates, such as neuroglobin and cytoglobin. Ngb resides in the central and peripheral nervous system of vertebrates. The exact function of Ngb is still uncertain, but there is evidence that it has a protective role related to the oxidative metabolism. Cygb is expressed in fibroblast-related cell types and distinct neurons. Cygb is possibly involved in collagen synthesis or O2 supply to distinct enzymes. Deoxygenated Hb and Mb display a socalled pentacoordinated heme, while Ngb and Cygb are hexacoordinated. Ngb and Cygb are widespread among vertebrates and occur in fishes, amphibians, reptiles, birds and mammals. Other globins appear to be restricted to certain vertebrate classes. In birds, an eye-specific globin has been identified, while globin Y is expressed in various tissues of Xenopus. GbX has only been identified in “lower” vertebrates, i.e. fishes and amphibians, but appears to have been lost in Amniota. Although the GbX sequence is highly conserved among fishes and amphibians, it displays only limited similarities to any other globin. In phylogenetic analyses GbX joins a clade consisting of Ngb, invertebrate nerve globins and the Hbs of the tunicate Ciona intestinalis. GbX is widely expressed in goldfish tissues but displays a more restricted localization in brain and eye of Xenopus. Hypoxia decreases the levels of GbX mRNA in adult zebrafish. Due to unique N- and C-terminal extensions, the GbX sequence is longer than that of a typical globin and spans,200 amino acids. Our results indicate that GbX is anchored in the cell-membrane by dual N-terminal acylation. While membrane-bound globins have previously been identified in some bacteria and the green shore crab, GbX is the first such globin-type discovered in vertebrates. In order to gain further insight into possible functional role of GbX, we also investigated heme coordination and reactivity towards O2 and the ability to form disulfide bonds. Our data provide evidence for a novel globin function in vertebrates that is associated with its membrane localization and unlikely to be involved O2 transport and storage. In previous studies using goldfish, a broad expression pattern of GbX in various non-neuronal tissues was observed. By contrast, GbX was preferentially expressed in the brain and eye of the clawed frog X. laevis. We identified high amounts of GbX mRNA in eyes and brain, which is consistent with the detection of GbX protein in selected brain and eye areas of D. rerio. We therefore conclude that the localization of GbX is likely conserved in fishes and amphibians. Whether the divergent pattern in goldfish is a unique feature of that species remains to be demonstrated.