Before the enactment of Senate Bill No. 899, apportionment was governed by former Labor Code sections 4663, 4750, and 4750.5.(Marsh v. Workers' Comp. Appeals Bd. (2005) 130 Cal.App.4th 906, 911-912 30 Cal. Rptr. 3d 598 (Marsh).) Former Labor Code section 4663 (repealed by Stats. 2004, ch. 34, 33) provided: "In case of aggravation of any disease existing prior to a compensable injury, compensation shall be allowed only for the proportion of the disability due to the aggravation of such prior disease which is reasonably attributed to the injury." Former Labor Code section 4750 (repealed by Stats. 2004, ch. 34, 37) provided: "An employee who is suffering from a previous permanent disability or physical impairment and sustains permanent injury thereafter shall not receive from the employer compensation for the later injury in excess of the compensation allowed for such injury when considered by itself and not in conjunction with or in relation to the previous disability or impairment. The employer shall not be liable for compensation to such an employee for the combined disability, but only for that portion due to the later injury as though no prior disability or impairment had existed." (Italics added.) Former Labor Code section 4750.5 (repealed by Stats. 2004, ch. 34, 38) provided, in relevant part: "An employee who has sustained a compensable injury and who subsequently sustains an unrelated noncompensable injury, shall not receive permanent disability indemnity for any permanent disability caused solely by the subsequent noncompensable injury." " 'Two sections, 4750 and 4663, applied to antecedent injuries. Former section 4750 relieved an employer from the burden of compensating an injured worker for disability attributable to a preexisting permanent disability or physical impairment. Former section 4663 did the same when an injured worker's disability was partially attributable to a preexisting disease or condition.' " (Marsh, supra, 130 Cal.App.4th at pp. 911-912.) Former section 4663 "left employers liable for any portion of a disability that would not have occurred but for the current industrial cause; if the disability arose in part from an interaction between an industrial cause and a nonindustrial cause, but the nonindustrial cause would not alone have given rise to a disability, no apportionment was to be allowed. " (Brodie, supra, 40 Cal.4th at p. 1326.) "Former section 4750 was interpreted as granting employees wide latitude to disprove apportionment based on prior permanent disability awards by demonstrating that they had substantially rehabilitated the injury. " (Brodie, supra, 40 Cal.4th at pp. 1326-1327.) Thus, "before the enactment of Sen. Bill 899, apportionment was 'concerned with the disability, not its cause or pathology.' " (Marsh, supra, 130 Cal.App.4th at p. 912; accord, Brodie, supra, 40 Cal.4th at p. 1326 under old apportionment rules "courts properly rejected apportionment of a single disability with multiple causes".)