For the NFBSI-16, cognitive debriefing results showed strong support for the content validity of the measure in a HR+/HER2- advanced breast cancer population; all of the patients in the sample for whom data were available understood the instructions and the majority of the items (14 out of 16), while the remaining two items were understood by at least 90% of the patient sample. This evidence concerns the gene ERBB2 and breast cancer.