In Drosophila embryogenesis, mitotic control undergoes a significant transition 
during the 14th interphase. Mitoses before interphase 14 run on maternal 
products, and occur in metasynchronous waves. Mitoses after interphase 14 
require zygotic transcription, and occur asyncronously in an intricate, highly 
ordered spatio-temporal pattern. Mutations at the string (stg) locus cause 
cell-cycle arrest during this transition, in G2 of interphase 14, yet do not 
arrest other aspects of development. This phenotype suggests that stg is 
required specifically for initiating mitosis. We describe the cloning of stg, 
and show that its predicted amino acid sequence is homologous to that of cdc25, 
a regular of mitotic initiation in the yeast S. pombe. In addition, we show that 
zygotic expression of stg mRNA occurs in a dynamic series of spatial patterns 
which anticipate the patterns of the zygotically driven cell divisions. 
Therefore we suggest that regulated expression of stg mRNA controls the timing 
and location of these embryonic cell divisions.