Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Formalin-fixed samples were routinely processed for histology and relative levels of cell proliferation and apoptosis were determined by counting the number of mitotic figures and apoptotic bodies among 2000 to 10,000 tumor cells in hematoxylin and eosin-stained sections using established morphological features, as previously described.(33,34) Apoptotic cells in xenograft tumors stained for cleaved caspase-3 by immunohistochemistry but this technique did not improve sensitivity or specificity (not shown).
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