Fenestration of the middle cerebral artery (MCA) is an unusual anomaly. We recognized and photographed MCA fenestration during an operation performed to clip an internal carotid artery aneurysm. The incidence of angiographic demonstration of this anomaly has been reported to be 0.26%. The clinical significance of fenestration of the vertebral and basilar arteries has primarily been the relationship of fenestration with other intracranial vascular anomalies. This is the first report of MCA fenestration associated with an aneurysm.

The in vivo function of the gene 2 protein of bacteriophage T7 has been examined. The gene 2 protein appears to modulate the activity of the gene 3 endonuclease in order to prevent the premature degradation of any newly-formed DNA concatemers. This modulation is not however a direct interacton between the two proteins. In single-burst experiments rifamycin can substitute for the gene 2 protein, allowing formation of fast-sedimenting replicative DNA intermediates and progeny phage production. This suggests that the sole function of the gene 2 protein is inhibition of the host RNA polymerase and that the latter enzyme directs or promotes the endonucleolytic action of the gene 3 protein.

A mutant of bacteriophage T7 is described which produces smaller plaques than the wild type and is defective in early phage DNA synthesis. The mutation is located in the Class II transcriptional region of the T7 genome together with all the other genes involved in phage DNA synthesis, but it could not be placed into any of the existing known T7 genes. DNA replication in strain R9 begins at the same time as for wild type although it proceeds very slowly until 15 minutes after infection, after which time DNA synthesis is apparently normal. It is concluded therefore that there are two types of DNA replication in phage T7, which differ with respect to their dependence on the mutant function. The change from one mode to the other is marked by the formation of folded, complex DNA inside the cell.

Fenestration of the middle cerebral artery (MCA) is an unusual anomaly. We recognized and photographed MCA fenestration during an operation performed to clip an internal carotid artery aneurysm. The incidence of angiographic demonstration of this anomaly has been reported to be 0.26%. The clinical significance of fenestration of the vertebral and basilar arteries has primarily been the relationship of fenestration with other intracranial vascular anomalies. This is the first report of MCA fenestration associated with an aneurysm.