Papilins are extracellular matrix proteins that share a particular, common order 
of types of protein domains. They occur widely, from nematodes to man, and can 
differ in the number of repeats of a given type of domain. Protein variety is 
increased by differential splicing of pre-mRNA. We report that Drosophila, which 
has a compact genome, expresses three splice variants of papilin during 
embryogenesis in developmentally defined patterns. These isoforms have different 
numbers of Kunitz and IgC2 domains. The papilin isoforms are expressed in 
specific cell types and contribute to different extracellular matrices in 
gastrulation folds, early mesoderm, heart formation, basement membranes, and 
elaboration of the excorporeal peritrophic membrane that lines the gut. This 
finding indicates an unexpectedly broad spectrum of different pericellular 
matrices in Drosophila embryos. Such papilin-containing matrices have 
developmental as well as functional significance, as we previously showed that 
both suppression of papilin synthesis and ectopic overexpression lethally 
disrupt organogenesis.