Objective      To explore the candidate disease-causing gene in a mental retardation family.       Methods      The proband’s chromosome karyotype was analyzed

and whole-exome sequencing was performed to screen the candidate gene. Sanger sequencing was used to check the result of whole-exome sequencing and investigate the genotypes in the patient’s family.       Results      The proband’s karyotype was normal (46

XY) in G-banding level. Using whole-exome sequencing method

1 455 single nucleotide polymorphisms and 187 indels were identified in the proband. Combined with the family information and the result of Sanger sequencing

ARHGAP4 gene with C2822T mutation in X chromosome was screened from the data and identified as disease-causing gene in the family.       Conclusion      C2822T mutation in ARHGAP4 gene on X chromosome may be related with the phenotype of mental retardation in this family.