def nRatioForOneVariant(chr_fa_seq, variant, size = 10):
    """
    Calculate the N ratio surrounding this variant's position 
    """
    left_ref_seq  = chr_fa_seq[max(0, variant.POS - size):max(variant.POS - 1, 0)]
    right_ref_seq = chr_fa_seq[variant.POS:variant.POS + size]  
    nr = 0.0
    for v in variant.ALT:
        seq = left_ref_seq + str(v) + right_ref_seq
        n = seq.upper().count('N') / float(len(seq))
        if n > nr:  # Just get the biggest n-ratio
            nr = round(n, 3)

    return nr