Phosphatidylinositol 3-phosphate [PtdIns(3)P] acts as a second messenger via the 
recruitment of diverse signalling proteins to various cellular compartments. 
Recent advances have highlighted the association of human diseases with 
mutations in phosphatases that regulate PtdIns(3)P levels. Myotubularin, the 
gene mutated in myotubular myopathy, functions as a lipid phosphatase with 
specificity for PtdIns(3)P. It is now apparent that there is an increasing 
family of proteins that are defined by their significant homology with 
myotubularin. The myotubularin-related gene family includes proteins that 
exhibit a lipid phosphatase catalytic motif, those that contain mutations of the 
critical catalytic residues, and at least one protein that functions as an 
adapter subunit for PtdIns(3)P-3-phosphatase activity. The present challenge is 
to understand how deregulation of phosphoinositide metabolism causes human 
disease.