Aplasia of the corpus callosum or aspermatogenesis is diagnosed by investigating existence or expression of the BT-IgSF gene with use of a primer for PCR or probe for hybridization comprising a DNA coding for a novel cell adhesion molecule (BT-IgSF) defined in the following (A) or (B) or a partial sequence thereof: (A) a protein comprising the amino acid sequence of the amino acid numbers 1 to 409 of SEQ ID NO: 2; (B) a protein comprising an amino acid sequence of the amino acid numbers 1 to 409 of SEQ ID NO: 2 including substitution, deletion, insertion or addition of one or several amino acids and having a function as a cell adhesion molecule.