Its expression has been shown to correlate with poor prognosis, treatment resistance and aggressive disease.22, 54, 55, 56 Relevance of ABCB1 expression in MM is supported by its strong, consistent and selective upregulation in several CFZ-resistant cell lines, as well as in a malignant PC patient sample that was analyzed before and after acquisition of CFZ resistance. This evidence concerns the gene ABCB1 and pachyonychia congenita.