Unsure of for loop

Unsure of for loop

Not sure what I'm doing wrong here. Any help would be appreciated. When I
enter a DNA sequence that contains a bad variable (like Z) I keep getting
a return True. Can someone point out why?
Thanks
def is_valid_sequence(dna):
""" (str) -> bool
>>> is_valid_sequence('ATCG')
True
>>> is_valid_sequence('AZT')
False
>>> is_valid_sequence('atcg')
False
Returns a boolean result based on whether dna is a valid
dna sequence.
"""
for char in dna:
if char in "TCGA":
return True
else:
return False