Most lysosomal diseases (LD) are inherited as autosomal recessive traits, but 
two important conditions have X-linked inheritance: Fabry disease and 
Mucopolysaccharidosis II (MPS II). These two diseases show a very different 
pattern regarding expression on heterozygotes, which does not seem to be 
explained by the X-inactivation mechanism only. While MPS II heterozygotes are 
asymptomatic in most instances, in Fabry disease most of female carriers show 
some disease manifestation, which is sometimes severe. It is known that there is 
a major difference among X-linked diseases depending on the cell autonomy of the 
gene product involved and, therefore, on the occurrence of cross-correction. 
Since lysosomal enzymes are usually secreted and uptaken by neighbor cells, the 
different findings between MPS II and Fabry disease heterozygotes can also be 
due to different efficiency of cross-correction (higher in MPS II and lower in 
Fabry disease). In this paper, we review these two X-linked LD in order to 
discuss the mechanisms that could explain the different rates of penetrance and 
expressivity observed in the heterozygotes; this could be helpful to better 
understand the expression of X-linked traits.