Although exposure to asbestos is the primary cause of mesothelioma, a slow-developing
cancer, researchers believe that infection by the SV40 virus is a contributing cause, since
in the United States 60 percent of tissue samples from mesotheliomas, but none from
healthy tissue, contain SV40

please see the above para.
whats the meaning of "since
in the United States 60 percent of tissue samples from mesotheliomas, but none from
healthy tissue, contain SV40" ?

i am confusing as the para has too many commas.
does it want to say 60 percent of tissue samples contain SV40


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