Def(Digestive-organ expansion factor)was firstly identified as a pan-endodermal enriched factor that is essential for the growth of digestive organs in zebrafish.Recently,we reported that loss of function of def leads to the accumulation of p53 protein in the nucleoli.Extensive studies have demonstrated that Def mediates p53 degradation independent of the ubiquitination-proteasome pathway but the activity of Calpain3.Peptide mapping showed Def N-terminus is necessary in promoting p53 degradation.We therefore are curious about Def N-terminus.When determining the molecular weight(MW)of Def and its deletions using Sodium dodecyl sulfate-polyacrylamide gel electrophoresis(SDS-PAGE),we noticed that Def N-terminus causes great higher apparent MW(~13 KDa).Then we treated Def N-terminus with phosphotase and glycanase and found that Def N-terminus is constitutively phosphorylated but not glycosylated.