Peptide tags for fluorescent labelling of proteins

A peptide comprising the amino acid sequence set forth in SEQ ID NO. 2:

• L1SX3AECAAX9EAACREX16X17ARAGGK23
• (SEQ ID NO:
• 2)
• wherein:
• X3 and X16 are independently selected from the group consisting of A, R, K, and H;
• X9 is R;
• and
• X17 is A, D, E, H, K, R, N, Q, or S;
• except that at least one of X3, X16, and X17 is not A, and when X17 is H, either X3 is not A or X16 is not A;
• or
• SEQ ID NO:
• 2 as defined above, further comprising one or two amino acid substitution(s), wherein the substitution(s) retain binding to a dimaleimide fluorescent labelling agent, wherein said substitution(s) are at position 1, 2, 4, 5, 7, 8, 10, 11, 12, 14, 15, 18, 19, 20, 21, 22, or 23 of SEQ ID NO:
• 2.