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 Igf2mutantmice, nutrient transporters are initially upregulated when there is first evidence of maldevelopment of the interhaemal compartment in the placental labyrinth layer (Constância et al. 2002; Sibley et al. 2004).
Category: