Previous studies have shown that expression of steady-state c-myb mRNA was regulated primarily by a block in intron 1 during transcription elongation. This study shows that the block site maps approximately 1700 bp from the start of the intron. Studies based on a reporter construct containing c-myb flanking region and intron 1 suggest that the flanking region is not important in the regulation of the cell type-specific expression of c-myb. RNA splicing of intron 1 may enhance the expression in a non-cell type-specific manner. A conserved intron domain comprising the block site is required for defining this site, but this function of the domain is independent of cell type. The cell type-specific regulation of c-myb transcription elongation is mediated by a 5' intron sequence. A mechanism for down regulation of c-myb gene expression by the block to transcription elongation has been proposed.