Alnylam had challenged the validity of five patents recently issued to Silence, and the first of those petitions has been thrown out by US authorities.
The US Patent Trial and Appeal Board found that Alnylam, valued at more than US$8bn, had failed to show enough evidence that the claims were invalid.
Silence said the PTAB’s decision not to review the issue of the patent “provides yet another vindication” of the strength of its intellectual property.
Decisions on the remaining patents are due over the next several months.
“We are very pleased with this favourable outcome in US patent office proceedings initiated by Alnylam,” said chief executive David Horn Solomon.
“We are hopeful that the remaining proceedings will result in similar outcomes and that Silence's patent position will be fully vindicated.”
Earlier this week, Silence told investors that European authorities are close to granting it another patent further protecting its gene silencing technology.
Silence has gone to war with Alnylam and others over the past year or so, claiming that they have been using technology covered by its patents in drugs that silence genes and may be fundamental in the war against cancer.
A court hearing in the UK between Silence and Alnylam – the company behind patisiran, the drug approved by the US Food and Drug Administration earlier this year – is expected to start in the UK in December.
Silence shares were down 1.2% to 120p.