Required for proper assembly of the eggshell. Synthesized in early stage 10 egg chambers. Cleavage generates S80, a 80 kDa species, during late stage 10 which is incorporated into the eggshell. During the latter stages of chorion formation, S80 is processed to a 60 kDa component, S60. Proteolytic cleavage of isoform FC106 generates 2 further products, S80 and S60. Allele FC1 is found only in strain Samarkand, allele FC3 only in strain Shahrinau and FC4 only in strain Dilizhan. Allele FC2 Is found in both strains Israel and Ghanghry.