Some people grow their myeloma cells (Sp2/0, Sp2/0-ag14, NS-0 and NS-1 cells for example) in 8-azaguanine or 6-thioguanineThe reason for this is to ensure that the cell line is 8-azaguanine or 6-thioguanine and HGPRT negative. Azaguinine and 6-thioguanine are competitors of guanine. This phenotype is necessary for the myeloma cells to be sensitive to the aminopterin in HAT (hypoxanthine-aminopterin-thymidine). Aminopterin blocks the nucleotide synthetic pathway and the HGPRT negative phenotype means that additionally the myeloma cells can not produce nucleotides via the salvage pathway and therefore will die unless fused. I suggest that the use of 8-azaguanine or 6-thioguanine at least some of the time is important to ensure the correct myeloma phenotype is maintained and only fused cells ultimately survive.