EDITORThere are a number of monogenic diseases with complex phenotypes which are clinically distinct but also overlap in phenotype with one or more other syndromes. If mutations in the same gene are responsible for causing the related syndromes, the diseases are allelic. Two diseases linked to Xq28, incontinentia pigmenti (IP, MIM 308310, Bloch-Sulzberger syndrome) and dyskeratosis congenita (DKC, MIM 305000, Zinsser-Cole-Engmann syndrome) show similarities in phenotype, although the modes of expression differ. Whereas IP is X linked dominant with embryonic lethality in males, the major form of DKC is X linked recessive. The gene responsible for causing DKC, DKC1, was recently identified¹ and maps about 20 kb proximal to the factor VIII gene, F8C.² Linkage analyses have provided evidence that the IP gene is located in the telomeric 2 Mb region of Xq28 distal to DXS52³ and lod scores of highest significance were found around F8C.^4 5 The physical map position of . . . [Full text of this article]