The “Proteins of young larvae-A and B (PYL-A and B)” are the major hemolymph proteins detected specifically at the young larval instars of the silkworm, Bombyx mori. The most dominant protein of them named PYL-A4 (29kDa in size) was found to exhibit polymorphic variation upon gel electrophoresis. Genetic analysis for PYL-A4 indicated that this protein is co-dominantly expressed by a gene residing on the 20th linkage group. Thus the gene was designated as “Young larval protein” with the gene symbol Pyl. A three-point test was performed by applying the genes oh (0.0) and Lp (6.2) as markers, and the Pyl locus was mapped to the 11.2 position on the 20th linkage group.