Karyotypically occult FIP1L1- platelet-derived growth factor receptor alpha and beta rearranged eosinophilic disorders respond to imatinib mesylate with almost 100% efficacy.     ///    "The FIP1L1- platelet-derived growth factor receptor alpha and beta-positive patients, and those with abnormal T-cell populations are currently the only clearly defined treatable subgroups of hypereosinophilic syndrome. "    ///    The FIP1L1- platelet-derived growth factor receptor alpha-negative responders to imatinib pose a question as to the existence of subentities with unrecognized tyrosine kinases-based mutation. 