FMR1 and fragile X syndrome: To this end, Fmr1-KO male (hemizygous, -/Y) and female (heterozygous, +/−) mice, together with their WT littermates, underwent behavioral tests for exploration, spatial memory, social interaction and communication at the juvenile age of 7–8 weeks, i.e., when most of the FXS-like behavioral alterations are absent or mild.