We report here a novel X-linked mutant, named faint lined (Fnl), which was discovered in the litter of an irradiated 3H1 male (Dr Bruce Cattanach, personal communication). The mutation is associated with fine dorsal striping in affected heterozygous females and prenatal lethality in males. Approximately 50% of Fnl/+ females die in utero and surviving animals have a reduced weight at birth and weaning. Histological studies failed to reveal the underlying basis of the phenotype or any gross structural abnormalities in internal organs (Fnl/+×Mus spretus) F1 affected females were backcrossed to 3H1 males and haplotype analysis positioned Fnl in the proximal region of the mouse X chromosome distal to Ant2 and proximal to Hprt. Therefore, Fnl lies within a defined conserved segment and its human homologue can be predicted to lie in the ANT2–HPRT region in Xq25. Further genetic resolution of co-segregating markers flanking Fnl established that Fnl lies in a 7·6±2·6 cM interval between DXMit50 and DXMit82.