Labor Code §4751
If an employee who is permanently partially disabled receives a subsequent compensable injury resulting in additional permanent partial disability so that the degree of disability caused by the combination of both disabilities is greater than that which would have resulted from the subsequent injury alone, and the combined effect of the last injury and the previous disability or impairment is a permanent disability equal to 70 percent or more of total, he shall be paid in addition to the compensation due under this code for the permanent partial disability caused by the last injury compensation for the remainder of the combined permanent disability existing after the last injury as provided in this article; provided, that either (a) the previous disability or impairment affected a hand, an arm, a foot, a leg, or an eye, and the permanent disability resulting from the subsequent injury affects the opposite and corresponding member, and such latter permanent disability, when considered alone and without regard to, or adjustment for, the occupation or age of the employee, is equal to 5 percent or more of total, or (b) the permanent disability resulting from the subsequent injury, when considered alone and without regard to or adjustment for the occupation or the age of the employee, is equal to 35 percent or more of total.

In the en banc decision, Escobedo v. Marshalls, 70 CCC 604 (2005), the Board found that under SB 899, "apportionment now can be based on non-industrial pathology, if it can be demonstrated by substantial medical evidence that the non-industrial pathology has caused permanent disability" (page 16) and summarized the interplay between Labor Code 4751 and 4663: "Applicant asserts that because SB 899 did not amend section 4751, relating to benefits payable by the Subsequent Injuries Benefits Trust Fund (“SIF”), this reflects a legislative intent that pathology is not one of the “other factors” upon which apportionment to non-industrial causes can be based. In essence, applicant asserts that if apportionment based on pathology were allowed, this would cause a flood of SIF benefit claims to be filed under section 4751. This is because, in applicant’s view, apportionment to pathology would decrease the percentage of disability for which the employer is responsible, while the overall level of disability would remain unchanged, leaving the SIF responsible for the difference. We disagree." (at page 17)
"…if an applicant’s non-industrial pathology causes apportionable permanent disability under section 4663 or 4664(a), then SIF benefits will not be payable under section 4751 unless the applicant demonstrates that the pathology was causing permanent disability prior to the subsequent industrial injury. Although this may mean that, in some cases, an injured employee will not get either permanent disability benefits or SIF benefits for the apportioned disability, this is not a major change from pre-SB 899 law, which held that an injured employee was not entitled to SIF benefits based on an asymptomatic disease process that was not labor disabling prior to the industrial injury. [citations omitted] In any event, it is an issue within the Legislature’s domain, not ours." (at pages 18-19)