The silkworm Bombyx mori produces massive amounts of silk fibroin in the posterior silk glands at the final stage of larval
growth (fifth instar).The mRNA of fibroin has been purified and identified by its simple repeating sequence (1). It is detectable only in the posterior silk glands. The number of copies of the fibroin gene was demonstrated to be one per haploid genome (gene concentration, 0.004%) in posterior silk glands as well as in other tissues (2, 3)