Selenophosphate synthetase 1 (SPS1) is an essential cellular gene in higher eukaryotes. Five alternative splice variants of human SPS1 (major type, Delta E2, Delta E8, +E9, +E9a) were identified wherein +E9 and +E9a make the same protein. The major type was localized in both the nuclear and plasma membranes, and the others in the cytoplasm. All variants form homodimers, and in addition, the major type forms a heterodimer with Delta E2, and Delta E8 with +E9. The level of expression of each splice variant was different in various cell lines. The expression of each alternative splice variant was regulated during the cell cycle. The levels of the major type and Delta E8 were gradually increased until G2/M phase and then gradually decreased. Delta E2 expression peaked at mid-S phase and then gradually decreased. However, +E9/+E9a expression decreased gradually after cell cycle arrest. The possible involvement of SPS1 splice variants in cell cycle regulation is discussed. (C) 2010 Elsevier Inc. All rights reserved.