Provided are substantially pure CD40 splice variants that include altered internal sequences and unique tail sequences relative to previously described CD40 protein sequences. Also provided are fragments of the CD40 splice variants comprising at least 10 amino acids including at least 4 amino acids of the unique tail sequence; the unique tail sequences, and homologues thereof having at least 10 amino acids and 90% identity and antibodies which bind to an epitope on such proteins are disclosed. Pharmaceutical compositions comprising the protein, antibodies, isolated nucleic acid molecule that encode such proteins and pharmaceutical composition comprising such nucleic acid molecules are also disclosed. The present invention additionally relates to recombinant expression vectors that include the nucleic acid molecules and host cells which comprise such recombinant expression vectors are disclosed. In vitro methods of detecting in a sample the presence and/or quantity of such proteins or transcript which encodes such proteins are disclosed as are kits and reagents for performing the methods. Methods of modulating CD40-CD154 interactions in an individual are disclosed.