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:In mammals, at least four groups of progenitors contribute to total neuronal output: radial glial cells (RGCs) and short neural precursors (SNPs) compose the VZ while intermediate progenitor cells (IPCs) and basal radial glia (bRG), which are derived from VZ cells, populate the SVZ (Noctor et al., 2001; Gal et al., 2006; Kowalczyk et al., 2009; Fietz et al., 2010; Hansen et al., 2010; Shitamukai et al., 2011; Wang et al., 2011).
Category: