def find_first_stop_codon(nucleotide_sequence):
    """
    Given a sequence of codons (expected to have length multiple of three),
    return index of first stop codon, or -1 if none is in the sequence.
    """
    n_mutant_codons = len(nucleotide_sequence) // 3
    for i in range(n_mutant_codons):
        codon = nucleotide_sequence[3 * i:3 * i + 3]
        if codon in STOP_CODONS:
            return i
    return -1