DNA evidence is a no-go zone in the trial.

In the last few minutes of questioning in the defamation trial involving the writer E. Jean Carroll on Thursday, Donald J. Trump’s lawyer Joseph Tacopina asked Ms. Carroll about the dress she wore on the day she says Mr. Trump raped her decades ago.

Judge Lewis A. Kaplan, who is presiding over the civil trial in Federal District Court in Manhattan, immediately dismissed the jury for the day and then asked Mr. Tacopina his intentions with his line of questioning, apparently expecting that he might be treading close a topic both sides have agreed not to touch — DNA.

Traces of DNA are often used in rape and sexual assault cases to identify an attacker, and Ms. Carroll’s lawyers have said in court papers that the dress in question has such evidence.

Ms. Carroll first requested in early 2020 that Mr. Trump provide a sample of his DNA for comparison. Mr. Trump repeatedly refused.

Nearly two years later, Mr. Tacopina told the court that his client was finally willing to provide a DNA sample, under certain conditions. Specifically, Mr. Trump offered his DNA in exchange for an appendix to a laboratory report that Ms. Carroll commissioned about genetic material on the dress.

Judge Kaplan called Mr. Trump’s offer a stalling tactic, noting in a pretrial ruling that his offer came after a deadline to disclose evidence had already passed. Judge Kaplan also said that the two sides had already agreed in a joint order that no DNA experts would be called as witnesses.

The judge also noted Mr. Trump had already had the report in question — except for the missing appendix — for three years.

And even if Mr. Trump provided DNA, Judge Kaplan said in his ruling, it might not matter for the trial. “It is possible that the results of further DNA analysis using Mr. Trump’s DNA sample would be entirely inconclusive,” he wrote.

The upshot is that DNA evidence cannot be brought up by either party at the trial, which explains why the judge stopped the proceedings when Mr. Tacopina started asking about the dress on Thursday.

Mr. Tacopina quickly assured the judge that he would not discuss DNA. “I’m actually shocked you think I would run afoul of the court’s order,” he said.