Convert an expression vector into a probability vector.
    Attribute:
        expression_vector (list): List of expression values.

def exp2prob(expression_vector):
    """Convert an expression vector into a probability vector.
    Attribute:
        expression_vector (list): List of expression values.
    """
    v = np.asarray(expression_vector)
    if np.sum(v) == 0:
        return np.zeros(len(expression_vector))
    else:
        return v / np.sum(v)