California Code, Probate Code - PROB § 20214

(a) If all property does not come into the possession of the trustee, the trustee is entitled, and has the duty, to recover from the transferees, the proportionate amount of the tax with which the transferees are chargeable under this chapter.

(b) If the trustee cannot collect from any transferee the amount of tax apportioned to the transferee, the amount not recoverable shall be equitably prorated among the other transferees who are subject to proration.