YFP(1) corresponds to an N-terminal portion (amino acids 1-158) of the YFP fluorescent protein. If the YFP(1) split fluorescent fragment is brought together with a complementary split fluorescent fragment in vivo, a functional fluorescent protein may be reconstituted. YFP(1) is designed for use in a fluorescent protein fragment complementation assay, which is used to study protein-protein interactions in vivo (PMID:15048128).