Mutations in the inhibitor of kappa light polypeptide gene enhancer in B‐cells, kinase gamma (IKBKG), also called nuclear factor‐kappaB (NF‐kB) essential modulator (NEMO), gene are the most common single cause of incontinentia pigmenti (IP) in females and anhydrotic ectodermal dysplasia with immunodeficiency (EDA‐ID) in males. The IKBKG gene, located in the Xq28 chromosomal region, encodes for the regulatory subunit of the inhibitor of kappaB (IkB) kinase (IKK) complex required for the activation of the NF‐kB pathway. Therefore, the remarkably heterogeneous and often severe clinical presentation reported in IP is due to the pleiotropic role of this signaling transcription pathway. A recurrent exon 4_10 genomic rearrangement in the IKBKG gene accounts for 60 to 80% of IP‐causing mutations. Besides the IKBKG rearrangement found in IP females (which is lethal in males), a total of 69 different small mutations (missense, frameshift, nonsense, and splice‐site mutations) have been reported, including 13 novel ones in this work. The updated distribution of all the IP‐ and EDA‐ID‐causing mutations along the IKBKG gene highlights a secondary hotspot mutation in exon 10, which contains only 11% of the protein. Furthermore, familial inheritance analysis revealed an unexpectedly high incidence of sporadic cases (>65%). The sum of the observations can aid both in determining the molecular basis of IP and EDA‐ID allelic diseases, and in genetic counseling in affected families. Hum Mutat 29(5), 595–604, 2008. © 2008 Wiley‐Liss, Inc.