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:A key in vitro finding was the increased release of cytochrome c, the molecular initiator of intrinsic apoptosis in many cancer cell lines, including: non-small cell lung carcinoma (H460) [16,17], squamous cell lung carcinoma (CH27) [18], hepatocellular carcinoma (HepG2, HCCM, and Hep3B) [24], nasopharyngeal carcinoma (NPC) [25,26], human neuroblastoma (SK-N-Be(2c) and SJ-N-KP) [27], premalignant keratinocytic (HaCaT) cells [33], skin fibroblast (Hs68) [33], epidermoid carcinoma (A431) [33], head and neck SCC (SCC25) [33], skin melanoma (A375) [33,34,36], and tongue squamous cell carcinoma (SCC-4) [40].
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