Bombyx mori, or the domesticated silkworm has been used for over 5000 years for the production of silk. Male silkworms are a bit better at producing high quality silk so breeders have been wanting to breed only males for a long time. Knowledge of the sex determining system is then convenient but the mechanism of sex determination in silkworms has eluded scientist for a long time.

All organisms on earth carry a blue-print of their construction in each cell. This blue-print is a genetic code written into either RNA or DNA and this code is transmitted to the next generation. Only viruses contain RNA, all other organisms contain DNA in every cell which is made up by four bases: guanine, cytosine, adenine and thymine. As seen in the figure, these bases form stable basepair bridges linking the two DNA strands together. Adenine only basepairs with thymine and guanine basepairs only with cytosine. All the cellular components required for copying DNA or translating the DNA code into proteins have only ever worked with the GATC bases.