The tissue expression patterns and relative staining intensity of β-gal are shown in
Figure 1 . Although some aspects of the expression have been reported elsewhere,
5 6 8 9 10 the mice have been backcrossed and bred for many generations, raising the possibility that expression has changed. As a result, we confirmed that the expected expression had been retained. Expression of β-gal in B10.A-GFAP-β-gal mice was indistinguishable from that of the line 16 mice from which they were derived
(Figs. 1A 1B 1C 1D 1E) , showing expression in astrocytes of brain and retina, optic nerve, and the epithelium of lens and cornea. There was no thymic expression in GFAP mice by RNase protection assay,
6 or by Xgal or ONPG assays. The B10.A-ROSA26 mice expressed β-gal in a wide variety of tissues, including the eye, brain, bone marrow, kidney, thymus, and spleen (not shown), at levels ranging from low to high
(Figs. 1F 1G 1H 1I 1J 1K 1L) . The hi-arr-β-gal mice continued to express at a high level in the retinal rod photoreceptor cells, at a much lower level in the pineal gland, and at a trace level in a small number of brain cells
(Figs. 1M 1N 1O) , as previously reported.
5 9 11 We did not find expression elsewhere, including the spleen, bone marrow, and thymus, which was also tested by RT-PCR.