Rac1 is a member of the Ras superfamily of small GTPases involved in signal transduction pathways that induce the formation of lamellipodia, stimulate cell proliferation and activate the JNK/SAPK protein kinase cascade. Here we describe that amplification by RT-PCR of the entire Rac1 coding sequence from a series of human adult and fetal tissues revealed beside the expected Rac1 cDNA, a variant product which contained additional 57 nucleotides between codons 75 and 76. This variant resulted in an in-frame insertion of 19 new amino acids immediately behind the switch II region, including two potential threonine phosphorylation sites for casein kinase II and protein kinase C. Primers designed within and downstream of the inserted nucleotide sequence allowed isolation of a genomic clone with intronic consensus sequences demonstrating that the insertion corresponds to a novel, yet undescribed exon 3b. This Rac1 splice variant, designated Rac1b, was predominantly identified in skin and epithelial tissues from the intestinal tract. Most notably, the expression of rac1b versus rac1 was found to be elevated in colorectal tumors at various stages of neoplastic progression, as compared to their respective adjacent tissues. We suggest that the 19 amino acid-insertion following the switch II region may create a novel effector binding site in rac1b, and thus participate in signaling pathways related to the normal or neoplastic growth of the intestinal mucosa.