ALDH1A1 and head and neck cancer: ALDH1A3 had two attractive features that justified further studies: ALDH1 activity marks human cancer progenitor cells16, 17, 18, 19, and ALDH1A3 has been recognized as the isoform conveying increased ALDH1 activity in lung, ovary, breast, head and neck cancer and melanoma20, 21, 22.