Previously, we had described a housekeeping like promoter that regulates expression of the SRC gene in many cell types. This promoter was found to be regulated by Sp1 and hnRNP-K. However, at that time we could find little evidence supporting a significant role for Sp3 in SRC activation. Interestingly, despite its first description some 12 years ago, a full length Sp3 clone has only recently been described. Previous mechanistic studies, including our own, employed a version of Sp3 that was significantly N-terminally truncated. In addition, several shorter Sp3 isoforms exist that result from internally initiated translation sites. To complicate matters further, all Sp3 isoforms can be modified by SUMO-1. Due to this newly emerging information few reports exist that systematically explore these various Sp3 isoforms (SUMOylated or not) and how they affect activity of specific mammalian promoters. We therefore undertook such a study to re-evaluate regulation of SRC by these various Sp3 isoforms. Using human and insect cells we found that the newly isolated full length version of Sp3 was only a weak to moderate activator of SRC. However, to our surprise, the more commonly used N-terminally truncated version of Sp3 was up to five times more active. We also found that mutations preventing SUMOylation of the shorter Sp3 isoforms were sufficient to convert them into potent transactivators of SRC. In contrast to other studies, however, we found that SUMOylation of full length Sp3 had little effect on its transcriptional properties. These results provide new insights into the complexity of Sp3 mediated transcription which appears to be highly dependent on the isoform bound, SUMOylation status and the promoter context.