Several commentators argue that there remains a gap between cerebellar long-term depression (LTD) studied in vitro and cerebellar functions such as motor learning. Recent developments in a gene-targeting technique for producing mutant mice defective in particular gene products has opened a new era for the study of synaptic plasticity and learning. There are three mutants in which cerebellar LTD are clearly deficient in vitro. Two of these mice show clear impairment in the conditioned eyeblink response, a paradigm known to involve the cerebellum. In contrast, one mutant with apparently normal cerebellar LTD showed no impairment in the conditioned eyeblink response. These new lines of evidence support the view that cerebellar LTD is a cellular basis for discrete motor learning.