The TU-144LL has been configured as a high-altitude research laboratory and will carry experiments in support of NASA's High-Speed Research (HSR) program. See the news release at NASA for more information. A total of 17 were built, one being lost at the Paris Air Show while avoiding a French jet hidden in the clouds.
The Tu144LL was in its shelter at Zhukovsky Air Base when I saw it. Note the NASA and Boeing badges by the front door.