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A two cell C. elegans embryo stained with antibodies against tubulin (red) and CeBUB-1, a spindle checkpoint protein (green). DNA is stained with DAPI (blue). Although a role for spindle checkpoint proteins in early embryonic cells has been controversial, an analysis of CeBUB-1, and other spindle checkpoint and kinetochore proteins, indicates that a spindle checkpoint is active in early C. elegans embryos (see Encalada et al on p. 1056 of this issue). In this image of a 2-cell stage embryo, CeBUB-1 is localized along the holocentric chromosomes during metaphase in the smaller posterior cell, where it is presumably actively monitoring chromosome attachment and tension. The larger anterior cell has already entered anaphase and, presumably because the spindle checkpoint is no longer active, CeBUB-1 is no longer detectable on the chromosomes. Thus the asynchrony of the early embryonic cell cycles illustrates both the presence of a functional spindle checkpoint up to metaphase, and the absence of the checkpoint after anaphase onset.