eCTD

The five modules

Module	Contents	Region
1	Administrative, labelling, forms	Region-specific
2	Summaries and overviews	Common
3	Quality (drug substance 3.2.S, drug product 3.2.P)	Common
4	Non-clinical study reports	Common
5	Clinical study reports	Common

Only Module 1 differs by region, which is the entire point of the harmonised structure: Modules 2 to 5 can be reused across agencies with far less rework than the pre-CTD world required.

Lifecycle is the part people underestimate

An eCTD is not a document you send. It is a cumulative record built from numbered sequences, each declaring the operation applied to every file: new, replace, append or delete. The agency's reviewing tool reconstructs the current state of the application from the whole history.

The consequence is that sequence integrity compounds. A lifecycle operation that points at the wrong predecessor does not fail loudly — it produces a submission that validates cleanly but renders a stale document as current. Errors made early persist through every subsequent sequence until someone notices, which may be years and dozens of sequences later.

Validation before submission

Submissions are checked against regional validation criteria, graded by severity. High-severity errors cause technical rejection — the application is not reviewed, and the clock does not start. Common causes are mundane: broken links, wrong file naming, checksum mismatches, missing leaf elements, incorrect study tagging.

This is why publishing is treated as a discipline in its own right rather than a formatting step, and why most organisations validate against agency tooling before transmitting rather than after.