We identified a novel splice variant of human SGT1 (SUGT1), a suppressor of the G2 allele of SKP1, by analysis of 8 human EST clones whose open reading frame encoded 365 amino acids. We termed this variant SGT1B (SUGT1B) and the original SGT1A (SUGT1A). The putative SGT1B and SGT1A proteins are 91% identical, and both contain a tetratricopeptide repeat (TPR) domain, two variable regions, a CS domain, and a SGS domain. The NCBI human genome database showed that SGT1B and SGT1A are located on chromosome band 13q14.13. SGT1B contains an additional 33 amino acids encoded by a region between exons 5 and 6 of SGT1A and lacks Ser110 of SGT1A. Immunoblotting using antibodies to N-terminal (amino acids 1-157) and C-terminal (amino acids 182-333) regions of SGT1A detected 2 bands whose sizes corresponded to those predicted for SGT1A and SGT1B. Overexpression experiments confirmed this finding. Additional immunoblot analysis demonstrated that both are highly expressed in human brain, liver, lung, and testis.