PSTPIP1 and pyoderma gangrenosum: There are at least three human diseases involving chronic multifocal sterile osteomyelitis that are caused by single gene mutations: (i) Majeed syndrome (LPIN2 mutations), (ii) deficiency of interleukin-1 receptor antagonist (DIRA, mutations in IL1RN) [26, 32, 33], and (iii) pyogenic arthritis, pyoderma gangrenosum, and acne syndrome (PAPA, mutations in PSTPIP1).