Whereas we were able to successfully target 1 of the 2 SLC6A4 alleles in diploid MCF-7 cells, and 3 of the 4 alleles in tetraploid HCC1954 tumor cells we were unable to derive a single clone from either cell line lacking functional SERT among hundreds of clones that were screened, implying that the gene is essential for the viability of these breast tumor cell lines (data not shown). The gene discussed is SLC6A4; the disease is neoplasm.