Amplified region (amplicon) around MLL gene is closely linked to the 11q23.3 commonly deleted region of neuroblastoma, which includes cancer-associated genes such as PHLDB1 (LL5A), BCL9L, FOXN5 (FOXR1), CBL, MFRP, and PVRL1 (Nectin) genes. FOXN6 (FOXR2) gene at human chromosome Xp11.21 is generated due to retrotransposition of ancestral Foxn5 gene during evolution. FOXN5 and FOXN6 orthologs share the common domain structure consisting of FN56 and Forhead-box (FOX) domains. Here, we identified and characterized mouse Foxn5 gene by using bioinformatics. Mouse Foxn5, consisting of six exons, was located within mouse genome sequences AC122428.4 and AC125129.5. Foxn5 locus at mouse chromosome 9B was synthenic to rat chromosome 8q22 and human chromosome 11q23.3. Mouse Foxn5 (180 aa) was C-terminally truncated compared with rat Foxn5 and human FOXN5. Mouse ‘Foxn5’ protein without FOX domain was generated due to a frame shift introduced by germ-line one-base deletion within exon 3. Mouse Foxn5 mRNA was expressed in embryonic germ cells and fertilized eggs. Germ-line mutation of Foxn5 gene in the mouse lineage might lead to divergent scenario of early embryogenesis between mouse and rat through the deregulation of Foxn5 target genes in mouse early embryos, and explain the difficulty in manipulation of rat embryonic stem (ES) cells based on the mouse equivalent system. This is the first report on identification and characterization of mouse Foxn5 gene as well as on species specific germ-line mutation of the Fox family gene.

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