Background and aims Systemic lupus erythematosus (SLE) and Sjögren’s syndrome are related by clinical and serological manifestations as well as genetic risks. Both diseases are more commonly found in women compared to men at a ratio of about 10 to 1. Common X chromosome aneuploidies, 47,XXY and 47,XXX, are enriched among men and women, respectively, in either disease suggesting a dose effect on the X chromosome.

Methods We examined cohorts of Sjögren’s syndrome or SLE patients with intensity plots of X chromosome single nucleotide polymorphism (SNP) alleles along with karyotype of selected subjects.

Results Among ˜2500 women with SLE we found three patients with a triple mosaic consisting of 45,X/46,XX/47,XXX. Among ˜2100 women with Sjögren’s syndrome, one patient had 45,X/46,XX/47,XXX with a triplication of the distal p arm of the X chromosome in the 47,XXX cells. Neither the triple mosaic nor the partial triplication were found among controls. In another Sjögren’s cohort, we found a mother-daughter pair with partial triplication of this same region of the X chromosome. The triple mosaic occurs in approximately 1 in 25 000 to 50 000 live female births, while partial triplications such are even rarer.

Conclusions Very rare X chromosome abnormalities are present among patients with either Sjögren’s or SLE, and may inform the location of a gene(s) that mediate an X dose effect as well as critical cell types in which such effect is operative.