The alternative splicing of a previously undiscovered 30 base exon confers a new level of polypeptide diversity on the N-CAM family of cell-surface glycoproteins. It results in the insertion of 10 amino acids into the fourth of five extracellular immunoglobulin-like folds. Each major size class of rat brain N-CAM mRNAs consists of members that contain or lack the exon. Furthermore, this splicing event is developmentally controlled: RNAs containing the inserted exon are expressed at extremely low levels (<3%) in embryonic brain but increase postnatally to 40%-45% of all N-CAM mRNAs in adult brain. Antibodies that recognize the alternative 10 amino acid segment react with a subset of N-CAM-expressing neurons in cultures of embryonic rat cells.
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