CA4 and retinitis pigmentosa: Interestingly, mutations in NHE3 have not been documented clinically; mutant CAIV causes retinitis pigmentosa (RP17) without pRTA having been reported in patients (Rebello et al., 2004) nor in mice with targeted CAIV disruption (Shah et al., 2005); and in patients with CAII mutations (discussed in greater detail below) causing a mixed pRTA and dRTA, the pRTA component can be mild suggesting that compensatory processes may be involved (Sly et al., 1985).