Katreese Barnes, a multi-instrumentalist, vocalist, composer and music producer who accompanied some of the biggest stars of the 1990s and 2000s and won two Emmys as musical director for “Saturday Night Live” — including one for a song (with an unprintable name) that became one of the earliest viral sensations on YouTube — died on Aug. 3 in Manhattan. She was 56 .

Her brother Jerry said the cause was breast cancer, which she had fought for the past 20 years .

She never recorded an album under her own name, but Ms. Barnes was known to musicians around New York City as a remarkably versatile studio musician. She played piano, electric keyboards and alto saxophone, produced albums and wrote songs.

From the early 1990s on, she recorded and toured with Roberta Flack, Sting, Chaka Khan and other household names .

But her work for television probably reached the widest audience. Starting in 2001, she served for about a decade as the pianist on “Saturday Night Live,” eventually becoming the show’s musical director.