def get_bases_composition(seq):
    """
    Proportion of A,T,G,C bases in a sequence
    """
    seq = seq.lower()
    n_A = seq.count('a')
    n_T = seq.count('t')
    n_G = seq.count('g')
    n_C = seq.count('c')
    return {'A': n_A, 'T': n_T, 'G': n_G, 'C': n_C}
