Beta multiplier reference
Zero current in the left-hand branch and zero current in the right-hand branch is a possible steady-state operating point. If you powered up this reference without a startup circuit the reference current would be zero.

MSU2 is a small pull-up, which turns on MSU3, which kickstarts the feedback loop in the beta reference. One the reference hits steady-state, MSU1 overdrives the pull-up and turns of MSU3, leaving the startup circuit high-impedance.