KBTBD4 and pachyonychia congenita: Integrating all these data, it appears that PPTID-C, similar to neoplasms referred as ‘transitional’ PPTID, ‘pleomorphic’ PPTID, Jouvet’s grade 2 PPTID, mostly correspond to PC and differ from the KBTBD4-mutated prototypical PPTID (Fig. 1).