Fanconi anemia (FA) is an autosomal recessive disorder characterized by bone 
marrow failure, cancer susceptibility, and a variety of developmental defects. 
The disease is clinically heterogeneous; eight different complementation groups 
(FA A-H) and, thus, genetic loci have been discovered. Two genes, FAA and FAC, 
have been cloned. Disease-associated mutations have been detected and rapid 
mutation screening makes possible the assignment of patients without resorting 
to time-consuming cell fusion and complementation analysis. Amplification of 
specific cDNAs from RNA followed by direct or indirect sequence analysis is a 
standard method for mutation detection. During the course of such examinations 
of the FAC gene, we have noted that frequently only one of the expressed alleles 
is successfully amplified. This can lead to false assignment of patients to a 
complementation group. As we report here, such cases can be rapidly clarified by 
retroviral gene transfer and complementation analysis.