What is the burden shifting instruction in alternative causation cases?

2 This theory--that all defendants have contributed to the harm, but that the degree of harm is uncertain--is distinct from the theory of "alternative causation" covered under BAJI No. 3.80, in which one defendant is a legal cause of the injury, but one or more are definitely not. (See Summers v. Tice (1948) 33 Cal.2d 80, 199 P.2d 1.) It is well established that a burden-shifting instruction is appropriate in alternative causation cases. (Ibid.; Sindell v. Abbott Laboratories (1980) 26 Cal.3d 588, 163 Cal.Rptr. 132, 607 P.2d 924.)