Hi Gage, When the time comes that there is progression while on Gilotrif, you will have a new biopsy to check for the T790M mutation. My onc told me they will do a liquid biopsy (blood draw) first and if that comes back negative (there can be false negative results) they will do a tissue biopsy. I have the same dx you do, EGFR+ Exon 19 deletion. Gilotrif is most effective if you have the Exon 19. If you google Tagrisso, you will find out more about it. No point in worrying about whether or not you will have the T790M. Enjoy the time Gilotrif is giving you! But educate yourself so you will know what options are out there, don't depend entirely on your oncologist to know everything. Are you on Facebook? If so there are a number of cancer organizations to follow, including Lungevity, that post a lot of information about new treatments. Lungevity also has a Group for patients with the EGFR mutation.