In addition to its crucial function in LTP, Arc is also essential for LTD. In Arc KO slices, low frequency stimulation of CA1 pyramidal neurons induced only a shortlasting and smaller LTD than was observed in wild-type controls [17]. In CA1 neurons, Arc is also required for LTD mediated by mGluR1s. Interestingly, this form of LTD does not require new Arc transcription but relies on translation of pre-existing dendritic Arc mRNA [29]. This translation is regulated by the Ca2+/Calmodulin-dependent eEF2K and FMRP [30]. Arc is also required for LTD in a different system in which LTD is induced in cultured Purkinje neurons with paired glutamate and depolarization [64]. This form of LTD is mediated by transcription that specifically requires the SRE site on the Arc promoter. The differing requirements for transcription of Arc in LTD in these two systems could potentially be the result of the different stimulations used to induce LTD, or the different cell types used, because mechanisms behind Hebbian plasticity can differ between cell types.