After confirming the specific targeting of Mlc1-tTA to astrocytes in the cortex, we employed Mlc1-dnEGFR double transgenic mice to study ErbB receptor function in astrocytes. TRE-dnEGFR transgenic mice encode a dominant-negative mutant of EGFR (dnEGFR) that is ectopically expressed under the control of a TRE with a cytomegalovirus minimal promoter (PminCMV) . DnEGFR is a truncated form of EGFR that lacks the intracellular kinase domain but retains the ability to bind to other ErbB family members . In this study, efficient suppression of ErbB receptor activity by dnEGFR was first verified by coexpressing dnEGFR with a comprehensive panel of ErbB receptors in HEK293 cells. When coexpressed with dnEGFR, the activity of each ErbB receptor was reduced 50–60%, indicating haploinsufficient function (Figure 1b and c). When not fed the tetracycline analog doxycycline (Dox), bi-transgenic Mlc1-dnEGFR mice expressed dnEGFR in Mlc1-controlled cells (Figure 1d). After failing to detect dnEGFR with available antibodies, we examined dnEGFR transcripts through real-time reverse transcription polymerase chain reaction (RT-PCR) and observed a 20- to 60-fold increase in the cerebral cortices of Mlc1-dnEGFR mice compared with those of littermate controls (Supplementary Figure S2a and b). Variations in the increase of dnEGFR transcripts in different Mlc1-dnEGFR mice may have been caused by varied insertion copies of the ectopic gene. Notably, dnEGFR expression in astrocytes during development did not change animal weight, body size or gross brain structures. In addition, immunostaining of GFAP or Acsbg1 in brain slices did not reveal significant differences in astrocyte number, distribution or size (Supplementary Figure S2c–g), suggesting that reduced ErbB receptor activity did not alter astrocyte development in Mlc1-dnEGFR mice.