Mice homozygous for the myelin-deficient (mld) mutation have an unusual phenotype in which the gene encoding myelin basic protein (MBP) is expressed at low levels and on an abnormal developmental schedule. In this report we describe the organization of the mld MBP locus, which results in this alteration of MBP expression. The mld MBP locus consists of two tandem MBP genes, with the upstream gene containing an inversion of its 3′ region. We also demonstrate that although there are low steady-state levels of MBP RNA in mld mice, the mid MBP locus is transcribed at a rate comparable to that of the wild-type MBP gene, indicating that the MBP transcripts are abnormally unstable.
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