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:Importantly, and consistent with previous studies (Browd et al. 2006; Kessler et al. 2009; Thomas et al. 2009), in the single Shh-dependent tumor identified, Ptch1 (a negative regulator of Shh) was expressed at very low levels, suggesting that loss of Ptch1 cooperated with MYCN to initiate rare GTML tumors driven by Shh.
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