Myelin-deficient (mld) mice present two tandem myelin basic protein genes, with the upstream gene containing an inversion (Popko et al., Neuron 1: 221, 1988). Detailed analysis of the transcriptional activity of various regions of the gene showed that all portions of the MBP gene are transcribed in mld mice. Consequently, we propose that the low levels of MPB mRNA observed in mld mice are due to post-transcriptional events.