(A) Down-regulation of XB130 enhanced spontaneous and induced cell death of WRO cells cultured with 10% FBS. Cells transfected with control or XB130 siRNA were treated with staurosporine (STS, 200 nM), or Fas antibody (FasAb, clone CH11, 100 ng/ml) for 24 h. Apoptosis was determined by flow cytometry using PI/Annexin V double staining. (B) Down-regulation of XB130 enhanced staurosporine-induced apoptosis in serum free condition in A549 cells. A549 cells transfected with control or XB130 siRNA were incubated in serum free medium with or without 200 nM STS for 24 h. n = 6. Mean ± SEM. *p<0.05 (compared with control siRNA). (C) XB130 can be phosphorylated by protein tyrosine kinases other than RET/PTC. Immunoprecipitation with anti-myc antibody in HEK293 cells transfected with myc-tagged XB130, along with RET/PTC3, EGFR, Src, Abl, or ERBB2, showed an increase in XB130 tyrosine phosphorylation using anti-phosphotyrosine antibody.