Napsin is found in two isoforms,napsin A and B,with highly homologous nucleotide sequences (91.2%). Napsin A appears to be a functional proteinase,predominantly expressed in lung and kidney. Napsin B is transcribed exclusively in cells related to the immune system and lacks an in-frame stop codon and is believed to be a pseudogene.. Napsin A is superior to TTF-1 in distinguishing primary lung ACA from other carcinomas (except kidney),particularly primary lung small cell carcinoma,and primary thyroid carcinoma..