Specificity: The antibody recognizes a transmembrane glycoprotein of 140 and 180 kD which has been identified as NCAM (Neural Cell Adhesion Module). It is involved in intercellular adhesion and play a role in outgrowth of neurites and the development of the nervous system. Several other isoforms have been identified which are expressed in a developmental and tissue specific pattern. Two major epitopes have been defined, NKI-nbl-1 (MON 9012) reacts with epitope 1 and 123C3 (MON 9006) reacts with epitope 2. At the international Workshop on SCLC antibodies 123C3 has been categorized as cluster 1 antibody (Stahel et al 1994). Most importantly, 123C3 can be used on routing formalin fixed paraffin section after retrieval. All cells in small cell carcinomas and carcinoids of the lung are strongly positive for 123C3. A minority of cases of other major types of lung carcinoma are sometimes positive as well: however this positivity is generally weak and focal. Adenoid cystic carcinomas of bronchial glands are strongly positive. Neuroblastomas and Wilms tumors are usually also staining strongly positive. In non small lung cell carcinomas, 123C3 staining has been associated with more advanced stage and a decreased survival after surgery (Kibbelaar et al. 1991). Positive staining with other tumors, include medullary thyroid carcinomas and some ovarian tumors. NCAM (CD56) is a marker for natural killer: 123C3 can be used to distinguish NK cells from other hematopoietic cell populations. Although ...