Abstract: We have built a comagnetometer that compares Helium-3 and Neon-21 precession frequencies using a SERF magnetometer. Spin-exchange interactions are suppressed with a sequence of RF pulses. I will discuss the operation of the system, and our current sensitivity, which has reached 7 nHz in 2000 seconds and below 1 nHz in a day. I will discuss several important experimental techniques that allowed us to reach that level, as well as some outstanding challenges and unexplained phenomena.