MFPs act as molecular switches by regulating alternative activities. If MFPs are limited in the cell, they only promote one pathway. There are cases where the same enzyme is able to promote or repress a specific path (a) and Rel (Msm) illustrates this example in the text. In other cases the MFP has the capacity to activate two alternative pathways and, if limited in its amount, it will activate one at the expense of another. In particular in the cases discussed here the proper regulation of the MFP activity is crucial. Discussed examples for this are the role of Xpd in steering the CAK activity and also LysRS.