Cellular proliferation during early embryo development is achieved by the s
erial cleavage of individual blastomeres into increasingly smaller cells, i
n the absence of cell growth. This simplified cell division process has fac
ilitated the study of the cell cycle and its regulatory pathways. The cell
cycle of mammalian cells is controlled by a number of mechanisms, including
the activity of cyclin-dependent protein kinase complexes. Numerous cyclin
proteins have been identified and these share structural and functional ch
aracteristics. For each of the A- and B-type cyclins, two subtypes have bee
n identified so far in mammals. However, in both cases the two subtype gene
s are expressed differentially, suggesting that they might have specific ro
les. The requirement for individual cyclin A and B proteins during early mo
use embryo development has been examined using gene-targeted deletion and i
mmunofluorescence techniques. These studies have shown that cyclin A1 is no
t essential for early embryonic development and cyclin A2 only becomes esse
ntial for development beyond the stage of implantation. Cyclin B1 is also e
ssential for development and its critical regulatory role during the meioti
c maturation of mouse oocytes will be considered. This review will discuss
the studies that have attempted to explain the possible redundancy between
the different cyclin subtypes.