Fanconi anemia (FA) is a rare inherited syndrome with diverse clinical symptoms 
including developmental defects, short stature, bone marrow failure, and a high 
risk of malignancies. Fifteen genetic subtypes have been distinguished so far. 
The mode of inheritance for all subtypes is autosomal recessive, except for 
FA-B, which is X-linked. Cells derived from FA patients are-by 
definition-hypersensitive to DNA cross-linking agents, such as mitomycin C, 
diepoxybutane, or cisplatinum, which becomes manifest as excessive growth 
inhibition, cell cycle arrest, and chromosomal breakage upon cellular exposure 
to these drugs. Here we provide a detailed laboratory protocol for the accurate 
assessment of the FA diagnosis as based on mitomycin C-induced chromosomal 
breakage analysis in whole-blood cultures. The method also enables a 
quantitative estimate of the degree of mosaicism in the lymphocyte compartment 
of the patient.