CycB expression in early embryos is diffusely concentrated at the anterior end of the embryo.

High levels of CycB transcripts are observed at times of rapid cell proliferation such as early and late embryo stages and at lower levels at other times. In preblastoderm embryos, CycB transcripts are detected throughout the cytoplasm with a higher concentration at the posterior end. During nuclear migration, CycB transcripts migrate in close association with the nuclei to the periphery and the mRNA at the posterior cap is incorporated into the forming pole cells. In somatic cells, signal decreases during blastoderm stages and increases again at gastrulation, at which time, transcripts are observed throughout the embryo. During germband retraction, CycB trancripts are observed in the developing nervous system but are no longer seen in epidermal cells that have completed their cell divisions.

CycB transcripts are concentrated around nuclei in syncytial embryos and then migrate with them to the cortex. They are concentrated in the regions around nuclei that are rich in microtubules. CycB transcripts were found to be concentrated at the posterior pole of early embryos and are thought to concentrate there during oogenesis. This posterior localization was abolished in mutants of stau, vas, spir, capu, and osk. Posterior localization of CycB transcripts is less striking in tud and vls mutants than in wild type embryos and indistinguishable from wild type in nos and pum mutants. No ectopic anterior expression of CycB was seen in BicD mutants.