Most mammary carcinomas induced in C3H mice by the mouse mammary tumour virus (MMTV) bear a new proviral insertion within a highly conserved locus on chromosome 15 called int-1. A transcriptional unit within this locus is inactive in all tested normal tissues but expressed at low levels in mammary tumours with proviral insertions positioned on either the 5' and 3' sides of the gene. Transcription of the proviruses proceeds away from int-1; thus an indirect mechanism appears to activate expression of this putative oncogene.

The retinoblastoma (Rb) gene is the prototype for a class of recessive human cancer genes in which loss of activity of both normal alleles is thought to be associated with tumorigenesis. Sixteen of 40 retinoblastomas examined with a complementary DNA probe shown to be the Rb gene had identifiable structural changes of the Rb gene including in some cases homozygous internal deletions with corresponding truncated transcripts. An osteosarcoma also had a homozygous internal deletion with a truncated transcript. In addition, possible hot spots for deletion were identified within the Rb genomic locus. Among those tumors with no identifiable structural changes there was either absence of an Rb transcript or abnormal expression of the Rb transcript. Comparison of the structural changes in the tumor cells and fibroblasts of certain patients provided support for Knudson's two-hit hypothesis for the development of retinoblastoma at the molecular level. The ability to detect germline structural deletions in fibroblasts from some patients with bilateral retinoblastoma also indicates that the isolated gene is useful for diagnostic purposes.

Tissue homeostasis and the prevention of neoplasia require regulatory co‐ordination between cellular proliferation and apoptosis. Several cellular proteins, including c‐myc and E2F, as well as viral proteins such as E1A, have dual functions as positive regulators of apoptosis and proliferation. The product of the retinoblastoma tumor suppressor gene, pRb, binds these proteins and is known to function in growth suppression. To examine whether pRb may function as a negative regulator of both proliferation and apoptosis, we analyzed apoptosis induced in transfected derivatives of the human osteosarcoma cell line SAOS‐2. Ionizing radiation induced apoptosis in a time‐ and dose‐dependent manner in SAOS‐2 cells, which lack pRb expression. In both a transient and stable transfection assay, SAOS‐2 derivatives expressing wild‐type (wt) pRb exhibited increased viability and decreased apoptosis following treatment at a variety of radiation doses. Expression in SAOS‐2 of a mutant pRb that fails to complex with several known binding partners of pRb, including E1A and E2F, did not protect SAOS‐2 cells from apoptosis. Radiation exposure induced a G2 arrest in SAOS‐2 and in derivatives expressing pRb. Inhibition of DNA synthesis and cell cycle progression by aphidicolin treatment failed to protect SAOS‐2 cells or pRb‐expressing isolates from undergoing apoptosis. Our data document a novel function for pRb in suppressing apoptosis and suggest that several proteins shown to induce apoptosis, including E1A, E2F and c‐myc, may do so by interfering with the protective function of pRb.

Avian leukosis virus (ALV), a slowly oncogenic retrovirus, induces in chickens a variety of neoplasms, including lymphoid leukosis and erythroblastosis. In lymphoid leukosis, a cellular oncogene, c-myc, is activated by the insertion of ALV LTR. We provide evidence that ALV utilizes a similar mechanism in erythroblastosis induction by activating a different cellular oncogene, c-erbB. We report the isolation, from leukemic erythroblast DNA, of a clone that represents the viral-cell junction fragment and carried the ALV LTR and part of the c-erbB locus. Restriction and sequence analyses reveal that the LTR is located upstream from the erbB coding region and is oriented in the same transcriptional direction; such a structure would be compatible with the promoter-insertion type of activation. Our findings provide a molecular explanation for the multipotency of slowly oncogenic retroviruses.