def findDistance(co1, co2):
    """
    Find the distance between two codons.

    @param co1: A C{str} of length three.
    @param co2: A C{str} of length three.
    @return: An C{int} distance in [0, 3].
    """
    return (co1[0] != co2[0]) + (co1[1] != co2[1]) + (co1[2] != co2[2])