The entire coding region was replaced by a construct containing a lacZ gene with a nuclear localization signal linked to the 5' end and a neomycin gene under the control of thymidine kinase promoter. The endogenous promoter remained intact and controlled the expression of the inserted lacZ gene. Immunohistochemical analysis failed to detect cyclin A2 protein in morphologically abnormal embyros, which were presumed to be homozygous.