Spiders use fibroins to make silk strands. This family includes tubuliform silk fibroins which are used to protect egg cases. This domain is a structural domain which is found in repeats of up to 20 in many individuals (although this is not necessarily the case). RP1 makes up structural domains in the N terminal while RP2 makes up structural domains in the C terminal. [1]

1: Solution structure of eggcase silk protein and its implications for silk fiber formation. Lin Z, Huang W, Zhang J, Fan JS, Yang D; Proc Natl Acad Sci U S A. 2009;106:8906-8911. PMID:19458259