The CMP-sialic acid synthetase (CSS) catalyzes the activation of sialic acid (Sia) to CNIP-Sia which is a donor substrate of sialytransferases. The vertebrate CSSs are usually localized in nucleus due to the nuclear localization signal (NLS) on the molecule. In this study, we first point out that a small, but significant population of the mouse CNIP-sialic acid synthetase (mCSS) is also present in cytoplasm, though mostly in nucleus. As a mechanism for the localization in cytoplasm, we first identified two nuclear export signals (NESs) in mCSS, based on the localization studies of the potential NES-deleted mCSS mutants as well as the potential NES-tagged eGFP proteins. These two NESs are conserved among mammalian and fish CSSs, but not present in the bacterial or insect CSS. These results suggest that the intracellular localization of vertebrate CSSs is regulated by not only the NLS, but also the NES sequences. (c) 2007 Elsevier Inc. All rights reserved.