The body of evidence in the R.Kelly kiddie-porn case could come down to this: a mole on the R&B raunchmeister’s back.

During opening arguments in his sex-tape trial Tuesday, the defense said the singer couldn’t possibly be the man who cavorted with a girl as young as 13.

Kelly, 41, has a “significant” mole on his lower middle back, one that’s been there since childhood, said lawyer Sam Adam Jr.

And the man seen performing a variety of explicit sex acts on the raw video made between 1998 and 2000?

“There is no mole on his back,” Adam told jurors in a packed Chicago courtroom.

“Robert Kelly is not on that tape. … Not a single witness can tell you that is him on the tape.”

The defense also claimed the girl on the tape is not the same person prosecutors have identified as the victim. They say the person on the tape is a prostitute.

Kelly’s supposed sex partner – who was between 13 and 15 at the time – also denies she’s on the video.

“The woman on that tape [is] paid with money,” Adam said.

When the tape was played for the jury, it showed a man handing a female money at the beginning of their encounter and leaving more at the end.

In between, there are sex acts on a bearskin rug. The young woman dances for the man, and he urges her, “Dance faster, baby.” She calls him “Daddy.”

A rapt jury watched the tape in dim light on a 4-foot screen while Kelly sat grim-faced a few feet away. Kelly was indicted on child porn charges six years ago after the tape – now “People’s Exhibit No. 1” – was sent to a Chicago Sun-Times reporter.

A prosecutor told the jury the tape was an R. Kelly production from start to finish, one that the Grammy winner “created, staged, produced and starred in.”

“A child doesn’t choose to be violated and placed on a videotape, a videotape that will live on forever – long after this child becomes an adult,” Cook County prosecutor Shauna Boliker said.

Kelly is best known for the song “I Believe I Can Fly” and sexually charged hits like “Bump N’ Grind.”

He married the late singer Aaliyah when she was just 15, and settled a lawsuit brought by the family of another underage girl he allegedly bedded. He faces up to 15 years if convicted.