KCNK9 Imprinting Syndrome is a severe neurological disorder caused by a mutation (G236R) of the two-pore domain potassium (K2P) channel, TASK3. The TASK3_G236R mutant channel displays considerably reduced current density compared to wildtype (WT) TASK3 channels (1, 2). In this study, we have investigated whether this difference arises from altered expression of the mutant channel at the membrane and whether the altered functional properties of the channel can be explained by this. Recombinant human TASK3 and TASK3_G236R mutant channels were tagged with green fluorescent protein and transiently expressed in tsA201 cells. The plasma membrane was stained with deep red cell mask and cells were viewed using confocal microscopy. Correlation data between the channel and the membrane were expressed as mean (± SEM) of the Pearson’s Correlation Coefficient (PCC) and the Mander’s Overlap Coefficient (MOC) and n = cells. Statistical analyses were with unpaired t tests. For TASK3 channels, PCC values were 0.55 ± 0.02 (n = 25) compared to 0.41 ± 0.04 (n = 24) for TASK3_G236R channels (p = 0.003). Similarly, MCC values were 0.84 ± 0.01 (n = 25) for TASK3 compared to 0.75 ± 0.02 (n = 24) for TASK3_G236R (p < 0.0001). Therefore, although the mutant channel was expressed at the membrane, expression levels were significantly less than for WT channels. The functional properties of TASK3_G236R mutant channels expressed at the membrane are altered for mutant channels compared to WT. Activation of Gαq-coupled, H1 receptors by the wake-active neurotransmitter histamine, has been shown to inhibit TASK3 channels and this underlies activation of cholinergic neurons by histamine in the basal forebrain (3). Whole-cell patch clamp recordings were obtained from recombinant TASK3 and TASK3_G236R channels both co-expressed with histamine (H1) receptors. Current through TASK3 channels co-expressed with H1 receptors was inhibited by 74 ± 3% (n = 4) following activation of the receptors by 20 mM histamine. Conversely, inhibition of TASK3_G236R channels by 20 mM histamine, was significantly attenuated (p < 0.001) and, indeed, a small activation of current of 11 ± 6% (n = 5) by histamine was observed. Thus, whilst less TASK3_G236R channels are expressed at the plasma membrane, this cannot explain all of the effects of the mutation on the channel, since inhibition of these TASK3_G236R channels by histamine receptor activation is significantly attenuated. Attenuation of histamine inhibition may contribute to the low arousal state seen in patients with KCNK9 Imprinting Syndrome (4). Furthermore, therapeutic strategies to increase TASK3_G236R channel expression at the membrane in KCNK9 Imprinting Syndrome will not overcome all of the functional consequences of the mutation.