CRISPR/cas9 genome editing is used to insert the following: loxP- exon 13-17 Ddx3x cDNA fragment- a bGH poly(A) transcription termination signal-loxP-a small region of intron 12-an intact exon 13 containing the R488H (arginine to histidine; CGC to CAC) missense mutation with an additional 2kb of mouse genomic DNA proximal to this construct and 1.6kb distal to the construct. The human R488H mutation is associated with intellectual development disorder, X-linked, syndromic, Snijers Blok type.