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:Heme/porphyrin transport proteins – Heme Carrier Protein 1 (HCP1), Feline Leukemia Virus subgroup C Cellular Receptor 1 (FLVCR1) and ATP-Binding Cassette subfamily G member 2 (ABCG2) – clearly affect flux of heme and porphyrin across the plasma membrane [21–23], while ATPBinding Cassette subfamily B member 6 (ABCB6) has been defined as the transporter of porphyrins into the mitochondria [24].
Category: