ovarian cancer cell line: phenotype disappears - (Aug/05/2013 )

Hello there,
Does anybody know what can cause the disappearance of a phenotype in cell lines? I am knocking down transporter genes in various ovarian cancer cell lines and it already happened twice that a phenotype (in colony formation assays) disappeared (I had fewer colonies in the knockdowns but later there was no difference anymore). Also, one of the cell lines had in general a lot more colonies than before and they also looked a bit different. Does anyone have an advice for me? By the way, the passage numbers were only a few passages apart.
Thanks, Angelika

Hmmm... mycoplasma?

No mycoplasma, bob1, but thanks for the reply! I am thinking if it might be off target effects?? Do you think this could be possible? Thanks!

Off target effects are very common for knockdown using siRNA/miRNA/shRNA  mediated KD, so it is entirely possible that this is the cause of what you are seeing.