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:There are several key mechanisms in tumorigenesis that may involve Hh/GLI signaling [11,13]; first, inactivating mutations in the negative regulators PTCH or SUFU, or activating mutations in the positive regulator SMO cause pathway activation in a cellautonomous and Hh ligand independent manner [5,26–28]; secondly, ligand-dependent autocrine mechanisms in which cancer cells both secrete and respond to Hh ligands causing cellautonomous pathway activation [29,30]; thirdly, paracrine mechanisms in which stromal cells are induced by Hh producing cancer cells [31–34].
Category: