Fanconi anemia (FA) proteins function in a DNA damage response pathway that 
appears to be part of the network including breast cancer susceptibility gene 
products, BRCA1 and BRCA2. In response to DNA damage or replication signals, a 
nuclear FA core complex of at least 6 FA proteins (FANCA, FANCC, FANCE, FANCF, 
FANCG and FANCL) is activated and leads to monoubiquitination of the downstream 
FA protein, FANCD2. One puzzling question for this pathway is the role of BRCA2. 
A previous study has proposed that BRCA2 could be identical to two FA proteins: 
FANCD1, which functions either downstream or in a parallel pathway; and FANCB, 
which functions upstream of the FANCD2 monoubiquitination. Now, a new study 
shows that the real FANCB protein is not BRCA2, but a previously uncharacterized 
component of the FA core complex, FAAP95, suggesting that BRCA2 does not act 
upstream of the FA pathway. Interestingly, the newly discovered FANCB gene is 
X-linked and subject to X-inactivation. The presence of a single active copy of 
FANCB and its essentiality for a functional FA-BRCA pathway make it a 
potentially vulnerable component of the cellular machinery that maintains 
genomic integrity.