p40 (p63 delta) is a marker recently determined to be highly specific for squamous basal cells in the immunohistochemistry (IHC) application. The current more routinely recommended marker, p63, appears to have less specificity compared to p40, especially on squamous cell tumors. The ability to differentiate between lung adenocarcinoma vs. squamous cell carcinoma is difficult and has bearing on the different therapeutic avenues for each subtype treatment. p63 antibody's ability to distinguish between the tumor types appears to be inferior when compared to p40. The ability to utilize an antibody probe for p40 as a squamous cell marker bolsters its use for future sub-classification of lung cancers, especially by immunohistochemical techniques.