Abstract
The mcf.2 transforming gene sequence has been located to the region between 29 and 61 kb 3' of the factor IX gene. Two unrelated haemophilia B patients who raise antibodies to infused factor IX ('inhibitors') have deletions in excess of 273 kb encompassing the factor IX and mcf.2 genes and a CG-rich island. We believe these patients show the first nullisomic deletion of a transforming gene to be reported. No clinical condition can be attributed to the loss of the mcf.2 gene.