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Frequent expression of the tumor antigen cak1 in squamous‐cell carcinomas

KI is a murine monoclonal antibody (MAb) derived from a hybridoma generated by the fusion of splenocytes of BALB/c mice immunized with a human ovarian tumor cell line, OVCAR‐3. This antibody reacts strongly with epithelial ovarian tumors and mesotheliomas. The antigen recognized by MAb Kl, designated CAKI, has recently been characterized as a 40‐kDa protein probably anchored to the cell surface by glycosyl‐phosphatidyli‐nositol. Using immunoperoxidase histochemical methods, we examined 37 squamous‐cell carcinoma (SqCC) samples from cervix, lung, esophagus and other origins, and 12 normal squamous epithelia of the cervix and esophagus for their reactivity with MAb KI. Of the SqCC specimens, 81% showed Kl reactivity with variable intensity, but none of 12 normal tissue samples of squamous epithelia did so. Two patterns of CAKI expression in tumor samples were found, i.e., a heterogeneous pattern with strong intensity, and a homogeneous pattern with weak intensity. Three carcinomas in situ of the larynx, vulva and esophagus were moderately positive with KI, suggesting that CAKI antigen may occur in the early stage of carcinogenesis of SqCC. The expression of CAKI was also compared with expression of CAI25, HER‐2/neu, p53 and P‐glycoprotein, and MAb KI was found to react most consistently with SqCC. Since KI reacts with a majority of cervical and esophageal carcinomas but has no detectable reactivity in normal epithelia of the cervix uteri and esophagus, MAb KI could be of value as a reagent to help distinguish between normal and neoplastic cells on sections as well as in cytological samples.