Because the LAP-tTA mice that were used to drive expression of transgenic human E2 have been demonstrated to produce liver specific expression [34], BCKDH enzyme activity and production of human E2 protein in liver of Lines A and 525A was examined. BCKDH enzymatic activity in liver from Lines A and 525A was ~6 and 5%, respectively, of the enzymatic activity present in control liver (Fig. 4C). As shown in Fig. 3B, the amount of human E2 (predicted MW~54 Kd) in these transgenic mice was quite variable between mice. In some of these transgenic mice, the level of human E2 was approximately equal to the amount of mouse E2 (~47 Kd) produced in liver of nontransgenic control mice. The observations that these near normal amounts of E2 protein result in only ~5–6% of normal BCKDH enzyme activity suggest that transgene derived E2 is functioning at a suboptimal level. It is probable that the c-myc tag at the carboxy terminus of the transgene derived human E2 interfered with enzymatic activity. This interpretation is consistent with previous studies which have revealed that the carboxy terminus of an analogous subunit of the pyruvate dehydrogenase complex is essential for subunit interactions [35] and insertion of a Hisx6 tag on the carboxy terminus of an analogous E. coli subunit interfered with normal subunit assembly [36]. It is also possible that human E2 was not fully functional when complexed with mouse E1 and E3 subunits. Human E2 shares ~88% identity to mouse E2 at the amino acid level. We also used western blot analysis to analyze expression of the E2 transgene in brain, kidney and muscle. As shown in Figure 3C, D and 3E respectively, expression of E2 is negligible in those tissues.