Hetero-pores

A hetero-oligomeric pore derived from Msp which comprises a narrowing having a net negative charge, wherein the hetero-oligomeric pore comprises at least one monomer comprising a narrowing forming region comprising an amino acid sequence having at least one amino acid substitution from a non-negatively charged amino acid to a negatively charged amino acid compared with an amino acid sequence of a reference narrowing forming region, wherein the reference narrowing forming region corresponds to amino acids 83 to 111 of SEQ ID NO:

• 2 and has the following substitutions:
• D90N, D91N, and D93N, and wherein none of the remaining monomers of the hetero-oligomeric pore has a sequence as set forth in SEQ ID NO:
• 2.