The staining of a paraffin-embedded fixed tissue section of PDX1735 relied on a combination of immunofluorescence for Ki67 and RNA ISH for RBPJ. RBPJ was selected as a marker of chr4p copy-number state because of the availability of a probe for RNA ISH and a consistent gene expression difference in our basal breast cancer PT/PDX panel between chr4p copy-neutral and deletion samples. This evidence concerns the gene RBPJ and breast cancer.