These Fmr1 knockout mice have a neomycin resistance cassette replacing exon 5 of the fragile X mental retardation syndrome 1 (Fmr1) gene. Defects in FMR1 cause Fragile X syndrome, one of the most common forms of inherited mental retardation. Fragile X syndrome is associated with an array of deficits in motor control, cognition, learning, and memory, although their overall brain morphology is generally normal. Male hemizygotes and female homozygotes are viable and fertile. Male hemizygotes show macroorchidism (enlarged testes). Macroorchidism in caused by an increased rate of Sertoli cell proliferation during embryogenesis which may be independent of FSH signalling. Male hemizygotes and females homozygotes also exhibit hyperactivity, learning deficits, altered dendritic spines of visual cortex pyramidal cells, and differences in a variety of behavioral tests. Comparison of homozygotes to wildtype littermates in hidden- and visible-platform water maze learning showed deficits in spatial learning and motor performance.

These Fmr1 knockout mice may be useful in applications related to the study of Fragile X Syndrome.

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