Targeting specific cancer cell types (reproduced from references 29 and 30 with permission). G92A expresses ICP4 under the control of an albumin promoter/enhancer sequence and preferentially replicates within albumin expressing cells. Albumin expressing human hepatomas grown in athymic mice are effectively treated by either one or two doses of G92A (left) whereas albumin non-expressing human prostate cancer cells are unaffected (right). In contrast, hrR3, an ICP6-deficient vector that is not promoter regulated is effective against either tumor type.