Oncogenic RNA (sarcoma) retroviruses — In classic cell-culture experiments, which mammalian cell types can these strongly transforming viruses convert into continuously proliferating transformed cells?

Introduction / Context:
Sarcoma-inducing RNA viruses (historically called RNA tumor viruses; modern classification: oncogenic retroviruses) are classic tools in cancer biology. In vitro, they can transform multiple mammalian cell types, producing hallmarks such as focus formation, loss of contact inhibition, anchorage-independent growth, and altered morphology.

Given Data / Assumptions:

• The question refers to transformation (not mere infection) leading to persistent, neoplastic-like growth in culture.
• Cell types listed are fibroblasts, myoblasts (muscle precursors), and iris epithelial cells.
• “Strongly transforming” retroviruses typically carry viral oncogenes (v-onc) that drive rapid transformation.

Concept / Approach:
Retroviral transformation depends primarily on the presence of a susceptible receptor and a permissive intracellular environment for v-onc expression, not on a single unique lineage. Numerous historical experiments have demonstrated transformation across diverse primary and continuous cell cultures, including fibroblasts (e.g., NIH/3T3), myoblasts, and various epithelial cells.

Step-by-Step Solution:

Verification / Alternative check:
Transformation has been observed using focus-formation assays, soft agar colony formation, and altered growth kinetics across many mammalian cell types when infected with acutely transforming retroviruses.

Why Other Options Are Wrong:

• Fibroblasts only: too narrow; other lineages are transformable.
• Myoblasts only: excludes well-documented fibroblast and epithelial transformation.
• Iris epithelial only: likewise too restrictive.

Common Pitfalls:
Confusing tropism (receptor usage) with absolute lineage specificity; many cell types are transformable if receptors and intracellular conditions are present.

Final Answer:
all of these