Siah proteins are vertebrate homologs of the Drosophila 'seven in absentia' gene. In this study, we characterized two splicing forms, Siah1a and Siah1b, of the Xenopus seven in absentia homolog 1 gene (Siah 1). Overexpression of xSiah1a led to severe suppression of embryo cleavage, while that of xSiah1b was not effective even at a high dose. Competition analysis demonstrated that co-expression of xSiah1a and 1b generated the same phenotype as overexpression of xSiah1a alone, suggesting that xSiah1b does not interfere with the function of xSiah1a. Since xSiah1b has an additional 31 amino acids in the N-terminus compared to xSiah1a, progressive truncation of xSiah1b from the N-terminus showed that inability of xSiah1b to affect embryo cleavage was associated with the length of the N-terminal extension of extra amino acids. The possible implication of this finding is discussed. (C) 2010 Elsevier Inc. All rights reserved.