Donald Trump's "birther" comments are no joke for Jerry Seinfeld.

The 56-year-old comedian has canceled his appearance at a benefit for Donald Trump's Eric Trump Foundation because the real estate mogul has been questioning whether President Obama was born in America.

Seinfeld was expected to attend the September 13 event benefiting St. Jude's Children's Research Hospital, but his rep tells CNN that Seinfeld has indeed pulled out of the engagement because of the remarks Trump has been making about the President.

Trump responded to Seinfeld's move with a letter, writing, "I just learned you canceled a show for my son's charity, The Eric Trump Foundation, which benefits the St. Jude's Children's Research Hospital (children with cancer) because of the fact that you think I am being very aggressive with respect to President Obama, who is doing an absolutely terrible job as our leader - just look at Libya, our economy, gas, food, and clothing prices and maybe you will understand what is going on!"

It's not so much that Seinfeld pulled out of the benefit, Trump goes on, although "the children of St. Jude are very disappointed" and "Bret Michaels, disgusted by what happened, is taking over."

"What I do feel badly about is that I agreed to do, and did, your failed show, 'The Marriage Ref,' even though I thought it was absolutely terrible," Trump wrote. "Despite its poor ratings, I didn't cancel on you like you canceled on my son and St. Jude. I only wish I did."

Seinfeld had agreed to do the benefit back in January, but according to his manager, he became "increasingly uncomfortable" with Trump's questioning of Obama's citizenship.

According to his manager, Seinfeld "feels this kind of demagoguery has no place in public discourse . . . He has respectfully withdrawn from the event, and is making a contribution both to the Eric Trump Foundation and to [St. Jude]."

Meanwhile, Trump's wife Melania has come to his defense, telling Joy Behar that her husband's possible presidential run is not a publicity stunt: