Random insertion of retroviral gene trap vector ROSA, Reverse Orientation with Splice Acceptor, which contains the beta-galactosidase gene and accompanying splice acceptor and neo sequences. The construct was inserted in reverse orientation in the retroviral vector pGen-. The transgene is controlled by an unknown endogenous promoter. lacZ is expressed in all tissues of the developing embryo and in most tissues of the adult transgenic mouse. (J:39814, J:79478)