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:br A recent work (Bhuiyan et al., 2004) in the SCNT bovine field demonstrated that transfected ear fibroblasts were less able to develop to the blastocyst stage than their non-transfected counterpart, regardless of passage number (early or late). More likely, discrepancies among results reported up to now are due to differences in vector type, transfection protocols (Bhuiyan et al., 2004), NT methods and donor cell culture conditions (Wells et al., 2003; Miyoshi et al., 2003). Furthermore, the site of gene(s) integration(s), the transgene used and its possible interference in endogenous gene expression could influence results (Hodges and Stice, 2003). Another aspect to consider is donor cell lines and genotypes, since Heyman et al. (2002) found that four different subcultures isolated from a single skin biopsy resulted in significantly different NT blastocyst rates (ranging from 5 to 30%), and Kühholzer et al. (2001) even found different morula and blastocyst rates among clonal colonies obtained from the same donor cell fetus.
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