A method of reducing the amount of slipping forward associated with polynucleotide binding protein-controlled movement of a target polynucleotide through a transmembrane pore, the method comprising: (i) identifying one or more amino acids in the polynucleotide binding protein which contact one or more amino acids in the transmembrane pore, wherein the polynucleotide binding protein comprises the sequence of SEQ ID NO: 24 or a variant thereof having at least 80% identity to the amino acid sequence of SEQ ID NO: 24 over the entire sequence, and wherein the transmembrane pore comprises seven or more monomers comprising the sequence of SEQ ID NO: 2 or a variant thereof having at least 80% identity to SEQ ID NO: 2 over the entire sequence; and (ii) modifying one or more of the amino acids in the polynucleotide binding protein which contact one or more amino acids in the transmembrane pore by amino acid substitution or insertion, whereby the modification reduces the amount of slipping forward associated with polynucleotide binding protein-controlled movement of the target polynucleotide through the transmembrane pore, wherein: (a) the one or more amino acids in the polynucleotide binding protein which contact one or more amino acids in the transmembrane pore comprises the amino acids at positions 1, 2, 3, 4, 5, 6, 51, 176, 177, 178, 179, 180, 181, 185, 189, 191, 193, 194, 195, 197, 198, 199, 200, 201, 202, 203, 204, 207, 208, 209, 210, 211, 212, 213, 216, 219, 220, 221, 223, 224, 226, 227, 228, 229, 247, 254, 255, 256, 257, 258, 259, 260, 261, 298, 300, 304, 308, 318, 319, 321, 337, 347, 350, 351, 405, 415, 422, 434, 437 and 438 in SEQ ID NO: 24 or at the corresponding positions in the variant thereof; and (b) the one or more amino acids in the transmembrane pore which contact one or more amino acids in the polynucleotide binding protein comprises the amino acids at positions 12, 14, 48, 52, 53, 54, 55, 56, 57, 58, 59, 60, 134, 135, 136, 137, 138, 139, 169 and 170 in SEQ ID NO: 2 or at the corresponding positions in the variant thereof. (i) identifying one or more amino acids in the polynucleotide binding protein which contact one or more amino acids in the transmembrane pore, wherein the polynucleotide binding protein comprises the sequence of SEQ ID NO: 24 or a variant thereof having at least 80% identity to the amino acid sequence of SEQ ID NO: 24 over the entire sequence, and wherein the transmembrane pore comprises seven or more monomers comprising the sequence of SEQ ID NO: 2 or a variant thereof having at least 80% identity to SEQ ID NO: 2 over the entire sequence; and (ii) modifying one or more of the amino acids in the polynucleotide binding protein which contact one or more amino acids in the transmembrane pore by amino acid substitution or insertion, whereby the modification reduces the amount of slipping forward associated with polynucleotide binding protein-controlled movement of the target polynucleotide through the transmembrane pore, wherein: (a) the one or more amino acids in the polynucleotide binding protein which contact one or more amino acids in the transmembrane pore comprises the amino acids at positions 1, 2, 3, 4, 5, 6, 51, 176, 177, 178, 179, 180, 181, 185, 189, 191, 193, 194, 195, 197, 198, 199, 200, 201, 202, 203, 204, 207, 208, 209, 210, 211, 212, 213, 216, 219, 220, 221, 223, 224, 226, 227, 228, 229, 247, 254, 255, 256, 257, 258, 259, 260, 261, 298, 300, 304, 308, 318, 319, 321, 337, 347, 350, 351, 405, 415, 422, 434, 437 and 438 in SEQ ID NO: 24 or at the corresponding positions in the variant thereof; and (b) the one or more amino acids in the transmembrane pore which contact one or more amino acids in the polynucleotide binding protein comprises the amino acids at positions 12, 14, 48, 52, 53, 54, 55, 56, 57, 58, 59, 60, 134, 135, 136, 137, 138, 139, 169 and 170 in SEQ ID NO: 2 or at the corresponding positions in the variant thereof. (a) the one or more amino acids in the polynucleotide binding protein which contact one or more amino acids in the transmembrane pore comprises the amino acids at positions 1, 2, 3, 4, 5, 6, 51, 176, 177, 178, 179, 180, 181, 185, 189, 191, 193, 194, 195, 197, 198, 199, 200, 201, 202, 203, 204, 207, 208, 209, 210, 211, 212, 213, 216, 219, 220, 221, 223, 224, 226, 227, 228, 229, 247, 254, 255, 256, 257, 258, 259, 260, 261, 298, 300, 304, 308, 318, 319, 321, 337, 347, 350, 351, 405, 415, 422, 434, 437 and 438 in SEQ ID NO: 24 or at the corresponding positions in the variant thereof; and (b) the one or more amino acids in the transmembrane pore which contact one or more amino acids in the polynucleotide binding protein comprises the amino acids at positions 12, 14, 48, 52, 53, 54, 55, 56, 57, 58, 59, 60, 134, 135, 136, 137, 138, 139, 169 and 170 in SEQ ID NO: 2 or at the corresponding positions in the variant thereof.