It appears there is a question as to who the father is, respond by calling or writing to the party on the letter, that you are willing to produce the child or children and take the DNA tests to resolve the issue.
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Contact the attorney representing you from the texas Attorney Generals Office. The Attorney General's Office likely filed the Paternity Action with the court to establish who is the biological father. The two of you were not married at the time of birth? If not, then there is no presumed father so parentity would need to be established. I hope this helps.