PURPOSE: To provide a coagulogen cDNA derived from horseshoe crab of North American origin and having a novel base sequence.

CONSTITUTION: A DNA expressed by an amino acid sequence of formula I (G is glycine; Q is glutamine; A is alanine; K is lysine; V is valine; R is arginine; L is leucine; C is cysteine; I is isoleucine; M is methionine; S is serine; F is phenylalanine; T is threonine; Y is tyrosine; D is aspartic acid; W is tryptophan; E is glutamic acid; H is histidine; N is asparagine; P is proline). It can be produced by separating mRNA and providing cDNA using a reverse transcriptase, etc. The starting raw material is e.g., a hemocyte cell, amebocyte, etc., of horseshoe crab of North American origin. Coagulogen can be produced by integrating the coagulogen cDNA in a manifestation vector and introducing the obtained plasmid into a host cell.