In this study, we identified and characterized a putative MBF1 transcription factor from Giardia. We found that Giardia MBF1 had a similar function in inducing transcription for cyst differentiation and it was stress-related, suggesting that the eukaryotic MBF1 family may have evolved before divergence of Giardia from the major eukaryotic line of descent. Here, EDF1 is linked to cyst.