p63 consists of two major isoforms-TAp63 and DNp63. These isoforms differ in the structure of the N-terminal domains. The TAp63 isoform (identified by anti-p63 antibody) contains a transactivation-competent ‘TA’ domain with homology to p53, which regulates the expression of the growth-inhibitory genes. In contrast, DNp63 isoform (identified by anti-p40 antibody) contains an alternative transcriptionally-inactive ‘DN’ domain, which antagonizes the activity of TAp63 and p53. The p40 (clone ZR8) antibody recognizes exclusively DNp63 but not TAp63. p40 is a squamous cell carcinoma ‘specific’ antibody.