"Efflux systems function via an energy-dependent mechanism (Active transport) to pump out unwanted toxic substances through specific efflux pumps." 
Many children with autism have had lab test results that show high levels of aluminum, mercury, copper and other metals, and a study by the California Department of Health Services found that "Air pollution [particularly metals and chlorinated solvents] boosts autism risk by 50 percent in newborns". Based on these 2 findings, metal toxicity certainly seems to be involved in the development of autism.
So if there is a genetic vulnerability, (which does seem to exist based on fraternal vs identical twin studies), it seems to me that genetic conditions that cause impaired metal efflux would be a logical starting point. A genetic metal efflux problem could explain why some children's bodies are not able to filter out the metals in the vaccines, and would therefore suffer health problems following vaccinations, while other children don't appear to have any problems.
The reports of many parents who witnessed their children regress soon after being vaccinated, in my opinion absolutely sounds like some type of injury. A healthy brain just does not lose previously aquired skills without being damaged by something. In fact, "The etiology of many neurodegenerative diseases has been only partly attributed to acquired traits, suggesting environmental factors may also contribute. Metal dyshomeostasis causes or has been implicated in many neurodegenerative diseases."
The NNii reports that children's aluminum levels exceed the minimal risk level after being vaccinated, but they're not concerned about that because "50-70% of injected aluminum [from vaccines] is excreted within 24 hours". That's a pretty big difference. So some children in their study still had 50% of the aluminum in their bodies the next day while others had only 30%. I would be very interested to know what percentage of this aluminum the children with autism were able to excrete. I would hypothesize that children with autism probably excreted even less aluminum than the group that was studied. (To see how much aluminum various vaccines contain, click here.)