p40 antibody (antibody recognizing ΔNp63 only) is an isoform of p63, is thought to function as a stem cell factor, responsible for maintaining cells in an uncommitted state with regenerative potential—a role that may be recapitulated in tumors derived from these cells. p40 is usually expressed in the basal or progenitor cell layer of stratified epithelia (e.g., squamous, urothelial, bronchial), basal cells of some glandular epithelia (e.g., prostate), and myoepithelial cells of breast and salivary glands, trophoblasts and thymic epithelial cells. In tumor tissues, p40 expression is specific for squamous cell carcinoma. p40 is equivalent to p63 in sensitivity for lung squamous cell carcinoma, but it is markedly superior to p63 in specificity, which eliminates a potential pitfall of misinterpreting a p63-positive adenocarcinoma or unsuspected lymphoma as squamous cell carcinoma. p40 appears to be a more reliable marker for squamous cell carcinoma.