 In most cases, pores formed by viroporins are nonselective or only weakly selective for particular ions or small molecules.However, some examples do show strong selectivity; examples include the influenza A virus M2 proton channel protein, which is highly selective for protons and is active at low pH, and the Chlorella virus Kcv protein, which is selective for potassium ions.An alternative mechanism is illustrated by the SARS-CoV E protein, which forms a pore that integrates membrane lipids whose polar head groups influence ion selectivity.