CCL21 and breast carcinoma: To ensure relevance to cancer, we next asked whether shortlisted genes were expressed in colorectal and breast cancer; two major tumor types in which serine metabolism has been studied8,9,29 (Extended Data Fig. 1a,b), which narrowed our list to 14 SLC-encoding genes that could be involved in serine transport and have relevance to cancer (Fig. 1g).