def onebp_inserts(seq):
    """Generate all variants of sequence with 1bp insert.

    Assumes unambiguous IUPAC codes A, C, G, T only.
    """
    seq = seq.upper()
    variants = set()
    for i in range(len(seq)):
        for s in 'ACGT':
            variants.add(seq[:i] + s + seq[i:])
    for s in 'ACGT':
        variants.add(seq + s)
    return variants
