RAG vs. fine-tuning vs. prompting — how to decide

Use a clean decision rule.

Prompting    — the knowledge is already in the model and stable, or you just
  need formatting/behavior; cheapest, try first
RAG          — the answer depends on external, changing, or proprietary facts
  that must be grounded and cited (a patient's report, current guidelines);
  this is the genomics case
Fine-tuning  — you need new behavior/style/format or to internalize a narrow
  domain's patterns, and you have good training data; not for injecting fresh
  facts (it's lossy and stale-prone)

They compose: fine-tune for domain tone, RAG for facts, prompt for control. For "explain my variant," RAG is correct because the facts are per-user and must be cited.