Cover legend: E14.5 rat ventral mesencephalic neural cells undergoing neuronal maturation after retroviral transduction. The cells have been dually transduced with two different murine Moloney leukemia retroviruses. The first retrovirus makes the cells express the transcription factor Lmx1a and the reporter gene DsRed2; the second retrovirus makes the cells express the transcription factor Ngn2 and the reporter gene eGFP. For more information, see the article by Roybon et al. in this issue (pages 3644–3656).