Koko, the gorilla who mastered sign language and appeared on Mister Rogers Neighborhood and numerous documentaries, died Tuesday morning at the age of 46, according to The Gorilla Foundation. Koko died in her sleep at the foundation’s preserve in California’s Santa Cruz mountains.

Koko was born July 4, 1971 at the San Francisco Zoo. Animal psychologist Francine “Penny” Patterson began teaching Koko to sign while Patterson was a graduate student in 1972, work which eventually became part of a project at Stanford University.

Koko was beloved worldwide, having appeared on the cover on National Geographic and in documentaries. Koko also appeared on an episode of Mister Rogers Neighborhood. According to The Gorilla Foundation, Koko had been watching Mister Rogers on television since she was just a baby gorilla, and the two became fast friends. Koko showed Mister Rogers how to sign “I love you.”

Koko also was mentioned on numerous TV shows, including Seinfeld. Betty White met Koko in 2004 and wrote about the gorilla in her book If You Ask Me. 

Actor William Shatner is among those who reacted following Koko’s death.