The invention relates to a mutant α-hemolysin (α-HL) pore which is useful for detecting one or more nucleotides by stochastic sensing. The pore is particularly useful for sequencing DNA or RNA. A molecular adaptor that allows detection of the nucleotide(s) is covalently attached to the pore. The pore is specifically modified to facilitate positioning of the adaptor and may be modified to facilitate covalent attachment.