Baculoviridae is a family of large DNA viruses that specifically infect insects.It contains four genera,Alpha-,Beta-,Gamma-,and Deltabaculovirus.Alphabaculovirus is further divided into Group Ⅰ and Ⅱ,and Group Ⅰ appears to be emerged most recently among all baculoviruses.Interestingly,there are 12 Group Ⅰ specific genes that are only found in this lineage.Studying these genes is helpful to understand how baculoviruses evolved.Here,we reported the functional analyzing results of ac73,a function unknown Group Ⅰ specific gene of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) which is the type species of baculovirus.The AC73 protein encoded by ac73 was found to be expressed during the late stage of infection and incorporated into the nucleocapsids of budded virus (BV) and occlusionderived virus (ODV).In infected cells,AC73 resided mainly in the ring zone region of the nucleus,and appeared to be assembled into occlusion bodies (OBs).The ac73 knockout and repaired viruses were constructed and studied by in vitro and in vivo infection.Although ac73 was not essential for BV and ODV or OB formation,the BV titer and viral infectivity in insect larvae of ac73 knockout AcMNPV decreased by about 5-8 and 3-4 fold compared to those of wild type virus,respectively,suggesting ac73 contributed to infectious BV production and viral infectivity in vivo.This research provides new insight into the function of this Group Ⅰ specific gene.