Hermanussen and Sippell (1985) reported a presumably X-linked recessive kindred. All carrier females had normal sexual and olfactory function. Hipkin et al. (1990) described male twins who were identical by DNA fingerprinting; one had full-blown manifestations of Kallmann syndrome, whereas the other showed normal sexual development and only hyposmia. In a second family, Hermanussen and Sippell (1985) observed 16-year-old twin sisters of whom one had retarded pubertal development and total anosmia, and the other, proven to be monozygotic by blood grouping and HLA typing, had undergone a normal menarche but showed total anosmia. The authors pointed out that sporadic cases of Kallmann syndrome have appeared only in families in which isolated anosmia (see 301700 , 107200 ) is present. They suggested that there is an acquired hypothalamic GnRH deficiency on the basis of preexisting anosmia.