PAK3-related mental retardation represents a rare cause of X-linked mental retardation associated with behavioural symptoms. So far, four families carrying PAK3 mutations have been reported, and in most cases PAK3 dysfunction resulted from missense mutations thought to affect either the catalytic or the N-terminal regulatory domain activity. Here, we report on a Tunisian family of X-linked moderate mental retardation with behavioural symptoms, common dysmorphic features, oro-motor impairment and secondary microcephaly. Linkage analysis showed that affected male subjects and obligate carrier female subjects share a common haplotype in the Xp21.31 - Xq23 region that contains the PAK3 gene. Direct sequencing of PAK3 coding exons and flanking intronic sequences allowed us to identify the first splice mutation in PAK3 gene located at the 5' end of intron 6 (c.276+4A>G), which results in a complete switch-off of the genuine donor splice site and an activation of a cryptic donor splice site (GTAAG) located four nucleotides downstream to the genuine one. RT-PCR experiments using the RNA from the patient's lymphoblasts showed that PAK3 transcripts contain four additional nucleotides that lead to a disruption of reading frame with a premature stop codon at position 128. Together with previously reported observations, our data further confirm that PAK3 mutations result in a specific form of X-linked mental retardation with fairly constant clinical features.