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[jefsummers]

You did not do what I suggested. More like

def filter_seq(dna):
    seek = 'ACGT'
    filtered = ''
    for nucleotide in dna:
        How would you check if the nucleotide is in seek? If it is, then
            add nucleotide to filtered
    return filtered

dna = 'ATATGCTGCNATATGCAC'
print(filter_seq(dna))

Do the part in the middle properly and it will work.

[perfringo]

For inspiration I can suggest to read another thread in this forum. There is example how any() can be used in this situation.