The present invention relates to reduced FK228 of the formula (I) ##STR00001## wherein R.sup.1 and R.sup.2 are the same or different and each is a hydrogen atom or a thiol-protecting group, or a salt thereof, and a histone deacetylase inhibitor containing this compound, an expression potentiator and a reactivation promoter of a transgene, and pharmaceutical agents containing them as active ingredients. The reduced FK228 or a salt thereof has a strong histone deacetylase inhibitory activity and this compound can be used for the prophylaxis or treatment of various diseases, in which histone deacetylation is involved, and for the gene therapy of such diseases.