The present invention provides a method of screening a subject for mutations in the CC2D2A gene that are associated with Joubert syndrome, an autosomal recessive form of mental retardation. The present invention also provides proteins that are associated with Joubert syndrome including proteins that includes an amino acid sequence that terminates in DHEGGSGMES (SEQ ID NO: 1). Also provided are nucleotide sequences encoding such proteins and methods of screening subjects to identify nucleotide sequences or proteins associated with Joubert syndrome.