(A) Reduced glucose uptake recorded in oocytes injected with mutant compared with WT cRNA. Shown are representative results recorded from 3 × 10 oocytes for each data point. *P < 0.05, **P < 0.01. (B) Glucose uptake as determined in 4 charges of erythrocytes from 3 patients of family PED1 carrying the Q282_S285del mutation (III-2, IV-1, IV-2; the index patient III-2 was measured twice and both of his sons once each) and in 10 charges from different normal controls. Plotted is the logarithm ln(1 – c_t/c_eq), with c_t being the assimilated radioactivity after time t and c_eq the 1 in the equilibrium (after 25 min), versus time yielding a linear function. The mean slope values of the linear fits in controls were –0.0280 ± 0.0018 s^–1 and in patients –0.0201 ± 0.0006 s^–1 (P < 0.05). Error bars are smaller than symbol size for the patients. (C and D) Using ion-selective electrodes, [Na⁺]_i and [K⁺]_i were recorded in oocytes, injected as indicated. [Na⁺]_i was increased and [K⁺]_i decreased for mutant compared with WT transporters. n = 6 for each group; P < 1 × 10^–6 (mutant versus WT), P < 1 × 10^–7 (mutant versus H₂O) (C); n = 4–5, P < 0.001 (mutant versus both WT and H₂O) (D). P values were calculated based on the potential differences between the ion-selective electrode and the conventional electrode. Therefore, error bars are not shown for the calculated ionic concentrations (see Supplemental Methods). (E) ⁸⁶Rb⁺ flux experiments in oocytes show increased values for the mutation compared with the WT. Shown are values from 3 × 8 oocytes for each condition. **P < 0.01. (F) Increased ⁸⁶Rb⁺ flux was also demonstrated in erythrocytes from 3 patients (III-2, IV-1, IV-2) compared with erythrocytes from 11 normal controls. ***P < 0.001. Representative experiments from individual batches of oocytes are shown (A and C–E). Individual values (C–F) and mean ± SEM (error bars in A, B, E, and F) are presented.