Sp1 and Sp3 are transcription factors involved in the regulation of numerous genes involved in oncogenesis. Sp3 is a bi-functional transcription factor with three different isoforms. Its bi-functional activity may in part be regulated by the relative expression of these isoforms. Northern blot analysis of Sp3 detects only a single transcript. Analysis of the known Sp3 cDNA sequence shows a high GC content and seven out-of-frame AUG codons located between the 5'-end of the mRNA and the two internal AUG initiation sites. This makes it highly unlikely that cap-recruited, translation initiation competent ribosomes could reach the internal start sites. A full human Sp3 expression construct was cloned. A bicistronic vector using Renilla and firefly luciferase showed internal ribosome entry site (IRES) activity in Sp3 RNA immediately 5' to the internal AUG sites. Also, the two smaller isoforms were translated more efficiently when full-length, uncapped transcripts were used, while the larger isoform was not translated. Mutants of Sp3 with AUG codons introduced 5' of the two internal start sites were generated. Results showed that they were unable to suppress the smaller isoforms in vitro. Furthermore, dual non-AUG to AUG mutations showed occlusion of the second introduced isoform (i.e., the isoform situated more 3') but not of the internally initiated isoforms. These experiments are consistent with IRES-mediated translation of the two smaller isoforms of Sp3. The presence of an IRES allows the possibility that Sp3 isoform ratios and activity are controlled at the translational level. This mechanism may allow cells to control the expression of numerous genes during mitosis and, thus, have profound effects on cell cycle regulation and tumorigenesis.