In affected members of a large consanguineous Tunisian family with Usher syndrome type IIC (USH2C; 605472 ), Hmani-Aifa et al. (2009) identified a homozygous 18131A-G transition in exon 85 of the GPR98 gene, resulting in a tyr6044-to-cys (Y6044C) substitution in a highly conserved residue in the second extracellular loop. The mutation was predicted to disrupt an interloop disulfide bridge, leading to an improperly folded loop and nonfunctional receptor. Heterozygous mutation carriers were unaffected. The family also segregated nonsyndromic retinitis pigmentosa-40 (RP40; 613801 ), caused by a homozygous mutation in the PDE6B gene ( ). One family member who was doubly homozygous for both mutations had a more severe ocular phenotype. Two family members who were doubly heterozygous for both mutations were unaffected at ages 82 and 65 years, respectively. Hmani-Aifa et al. (2009) commented that consanguinity can increase familial clustering of multiple hereditary diseases within the same family. The family had originally been reported by Hmani et al. (1999) .
The G α subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity, allowing it to re-associate with G βγ and starting a new cycle. A group of proteins called Regulator of G protein signalling (RGSs), act as GTPase-activating proteins (GAPs), are specific for G α subunits. These proteins accelerate the hydrolysis of GTP to GDP, thus terminating the transduced signal. In some cases, the effector itself may possess intrinsic GAP activity, which then can help deactivate the pathway. This is true in the case of phospholipase C -beta, which possesses GAP activity within its C-terminal region. This is an alternate form of regulation for the G α subunit. However, it should be noted that such G α GAPs do not have catalytic residues (specific amino acid sequences) to activate the G α protein. They work instead by lowering the required activation energy for the reaction to take place.