). Little is known, however, regarding sequences that mediate posttranscriptional RNA stability. Characterization in our laboratory of a mutant murine B lymphoma, M12.C3, revealed a posttranscriptional defect affecting the synthesis of a major histocompatibility complex class II gene (AI3d) whose product normally controls both the specificity and magnitude of the immune response. Molecular studies revealed that the mutation responsible for diminished Apd gene expression was an intronic deletion of 10 base pairs (bp) located 99 bp 5' of the third exon. This deletion lies in a region not known to be critical for accurate and efficient splicing. Furthermore, sequence analysis of amplified Aj8-specific cDNA demonstrated that the small number of A13d transcripts produced in the mutant cells was correctly spliced. It appears that the mechanism by which this intronic 10-bp deletion acts to decrease RNA stability is unlikely to be at the level of RNA splicing.Significant progress has been achieved in identifying transcriptionally active DNA sequences; however, relatively little is known regarding sequences that act posttranscriptionally to regulate gene expression. Recently, it has become apparent that many genes are regulated at the posttranscriptional level (8,24,36). One notable instance involves the immunoglobulin heavy-chain gene. Gerster et al. (8) have found that the activity of the immunoglobulin heavy-chain gene enhancer and hence the transcription rate of the immunoglobulin heavy-chain gene are both high in pre-B-cell lines, although mRNA levels in these cells are relatively low. These studies indicate that increased levels of immunoglobulin gene expression in plasma cells are dependent upon posttranscriptional mechanisms. Furthermore, the induction of human beta and gamma interferon expression (22,36), the developmental regulation of the psp gene in the mouse parotid gland (33), and the tissue-specific expression of glyceraldehyde 3-phosphate dehydrogenase in rat tissue (21) all appear to be partially controlled by posttranscriptional events.The cis-acting sequences that act posttranscriptionally to regulate gene expression are not known for many of these cases. However, an AT-rich sequence has recently been identified that causes selective mRNA degradation when introduced into the 3' untranslated region of the ,B-globin gene, which normally produces a stable mRNA (32). This AT-rich sequence has been found in the 3' untranslated region of a number of posttranscriptionally regulated genes, including the human beta and gamma interferons as well as the bovine interleukin-2 gene (2,25,32 this 3' AU-rich sequence may be involved in these events (24).The sequence requirements, if any, for intranuclear stability or transport of heterogeneous nuclear RNA (hnRNA) to the splicing machinery are unknown, although consensus splice site sequences have been identified. Recently, an intronic point mutation within the c-Ha-ras oncogene has been shown to cause a 10-fold increase in expression that may be mediated by increased ...