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:Few studies (Zakhartchenko et al., 2001; Arat et al., 2001; Bhuiyan et al., 2004) have evaluated the effects of long-term culture and genetic manipulation of adult somatic donor cells for NT in bovines.
Category: