My onc explained that to me when mine returns. First time around I was triple positive and the chemo they gave me was designed to interfere with those receptor sites. I also took a vaccine that was designed specific kill cancer cells that over expressed Her2 mildly. I didn't have enough Her2 receptor sites to qualify for Herceptin. Because chemo is so target specific it isn't uncommon for recurrences to have different receptors. He wasn't the least shocked with my recurrence was ER+ and negative everything else. This latest recurrence wasn't biopsied at all because none of the new tumors are easily accessible for biopsy. So who knows what the heck it is. We chose a treatment method that is just out of clinical trails. So, if it works great. If it doesn't work we already have a back up plan. Maybe by the time we know it isn't working one of the tumors in my lungs will be large enough to biopsy.
Hope that helps
Take GOoD care