Despite her monumental reputation, the Mona Lisa was a mere thirty-one inches by twenty-one inches -- smaller even than the posters of her sold in the Louvre gift shop. She hung on the northwest wall of the Salle des Etats behind a two-inch-thick pane of protective Plexiglas. Painted on a poplar wood panel, her ethereal, mist-filled atmosphere was attributed to Da Vinci's mastery of the sfumato style, in which forms appear to evaporate into one another.

Langdon, now having made it clear to Sophie that he had no intention of leaving, moved with her across the Salle des Etats. The Mona Lisa was still twenty yards ahead when Sophie turned on the black light, and the bluish crescent of penlight fanned out on the floor in front of them. She swung the beam back and forth across the floor like a minesweeper, searching for any hint of luminescent ink.

Since taking up residence in the Louvre, the Mona Lisa or La Jaconde as they call her in France -- had been stolen twice, most recently in 1911, when she disappeared from the Louvre's "satte impénétrable" -- Le Salon Carre. Parisians wept in the streets and wrote newspaper articles begging the thieves for the painting's return. Two years later, the Mona Lisa was discovered hidden in the false bottom of a trunk in a Florence hotel room.

Walking beside her, Langdon was already feeling the tingle of anticipation that accompanied his face-to-face reunions with great works of art. He strained to see beyond the cocoon of purplish light emanating from the black light in Sophie's hand. To the left, the room's octagonal viewing divan emerged, looking like a dark island on the empty sea of parquet.

The Mona Lisa's status as the most famous piece of art in the world, Langdon knew, had nothing to do with her enigmatic smile. Nor was it due to the mysterious interpretations attributed her by many art historians and conspiracy buffs. Quite simply, the Mona Lisa was famous because Leonardo da Vinci claimed she was his finest accomplishment. He carried the painting with him whenever he traveled and, if asked why, would reply that he found it hard to part with his most sublime expression of female beauty.

Langdon could now begin to see the panel of dark glass on the wall. Behind it, he knew, in the confines of her own private cell, hung the most celebrated painting in the world.

No mystery at all, Langdon thought, moving forward and watching as the faint outline of the painting began to take shape. No mystery at all.

In actuality, the painting was a surprisingly ordinary sfumato portrait. Da Vinci's veneration for this work, many claimed, stemmed from something far deeper: a hidden message in the layers of paint. The Mona Lisa was, in fact, one of the world's most documented inside jokes. The painting's well-documented collage of double entendres and playful allusions had been revealed in most art history tomes, and yet, incredibly, the public at large still considered her smile a great mystery.

Even so, many art historians suspected Da Vinci's reverence for the Mona Lisa had nothing to do with its artistic mastery.

Most recently Langdon had shared the Mona Lisa's secret with a rather unlikely group -- a dozen inmates at the Essex County Penitentiary. Langdon's jail seminar was part of a Harvard outreach program attempting to bring education into the prison system -- Culture for Convicts, as Langdon's colleagues liked to call it.

"He screwed it up?" one of the inmates asked.

Standing at an overhead projector in a darkened penitentiary library, Langdon had shared the Mona Lisa's secret with the prisoners attending class, men whom he found surprisingly engaged -- rough, but sharp. "You may notice," Langdon told them, walking up to the projected image of the Mona Lisa on the library wall, "that the background behind her face is uneven." Langdon motioned to the glaring discrepancy. "Da Vinci painted the horizon line on the left significantly lower than the right."

Langdon chuckled. "No. Da Vinci didn't do that too often. Actually, this is a little trick Da Vinci played. By lowering the countryside on the left, Da Vinci made Mona Lisa look much larger from the left side than from the right side. A little Da Vinci inside joke. Historically, the concepts of male and female have assigned sides -- left is female, and right is male. Because Da Vinci was a big fan of feminine principles, he made Mona Lisa look more majestic from the left than the right."

This elicited a hearty round of laughs. Langdon considered offering an etymological sidebar about the word hermaphrodite and its ties to Hermes and Aphrodite, but something told him it would be lost on this crowd.

Langdon winced. "Historians don't generally put it quite that way, but yes, Da Vinci was a homosexual."

"I heard he was a fag," said a small man with a goatee.

"You mean like chicks with dicks?" someone called.

"Is that why he was into that whole feminine thing?"

"Hey, Mr. Langford," a muscle-bound man said. "Is it true that the Mona Lisa is a picture of DaVinci in drag? I heard that was true."

"Actually, Da Vinci was in tune with the balance between male and female. He believed that ahuman soul could not be enlightened unless it had both male and female elements."

"It's quite possible," Langdon said. "Da Vinci was a prankster, and computerized analysis of the Mona Lisa and Da Vinci's self-portraits confirm some startling points of congruency in their faces. Whatever Da Vinci was up to," Langdon said, "his Mona Lisa is neither male nor female. It carries a subtle message of androgyny. It is a fusing of both."

"You sure that's not just some Harvard bullshit way of saying Mona Lisa is one ugly chick."

Now Langdon laughed. "You may be right. But actually Da Vinci left a big clue that the painting was supposed to be androgynous. Has anyone here ever heard of an Egyptian god named Amon?"

Langdon was stunned.

"Hell yes!" the big guy said. "God of masculine fertility!"

The question met with several seconds of silence.

"It says so on every box of Amon condoms." The muscular man gave a wide grin. "It's got a guy with a ram's head on the front and says he's the Egyptian god of fertility." Langdon was not familiar with the brand name, but he was glad to hear the prophylactic manufacturers had gotten their hieroglyphs right. "Well done. Amon is indeed represented as a man with a ram's head, and his promiscuity and curved horns are related to our modern sexual slang'horny.'"

"No shit," Langdon said. "And do you know who Amon's counterpart was? The Egyptian goddess of fertility?"

"It was Isis," Langdon told them, grabbing a grease pen. "So we have the male god, Amon." He wrote it down. "And the female goddess, Isis, whose ancient pictogram was once called L'ISA." Langdon finished writing and stepped back from the projector.

"No shit!"

At first Langdon saw nothing. Then, as he knelt beside her, he saw a tiny droplet of dried liquid that was luminescing. Ink? Suddenly he recalled what black lights were actually used for. Blood.

"Mona Lisa… holy crap," somebody gasped.

"He wouldn't have come here without a reason," Sophie whispered, standing up. "I know he left a message for me here." Quickly striding the final few steps to the Mona Lisa, she illuminated the floor directly in front of the painting. She waved the light back and forth across the bare parquet.

AMON L'ISA "Ring any bells?" he asked.

"My grandfather was here," Sophie said, dropping suddenly to her knees, now only ten feet from the Mona Lisa. She pointed the black light tentatively to a spot on the parquet floor.

His senses tingled. Sophie was right. Jacques Saunière had indeed paid a visit to the Mona Lisa before he died.

Langdon nodded. "Gentlemen, not only does the face of Mona Lisa look androgynous, but her name is an anagram of the divine union of male and female. And that, my friends, is Da Vinci's little secret, and the reason for Mona Lisa's knowing smile."

At that moment, Langdon saw a faint purple glimmer on the protective glass before the Mona Lisa.

Reaching down, he took Sophie's wrist and slowly moved the light up to the painting itself.

"There's nothing here!"

They both froze.

On the glass, six words glowed in purple, scrawled directly across the Mona Lisa's face.