UPDATE 1-Roche offering to buy Illumina for about $5.7 bln

* To offer $44.50 per share in cash

* Offer price at 18 pct premium to Tuesday close

* Says will commence a tender offer

Jan 24 (Reuters) - Swiss drugmaker Roche Holding AG said it is offering about $5.7 billion in cash to buy U.S. gene sequencing device maker Illumina Inc, in what could potentially become a hostile takeover bid.

Roche said it would offer to acquire all shares of Illumina for $44.50 per share in cash, an 18 percent premium to Illumina's Tuesday close price of $37.68 on the Nasdaq.

The Swiss drugmaker said it would commence a tender offer as Illumina was not willing to negotiate a transaction.

"Roche has made multiple efforts to engage with Illumina in order to reach a negotiated transaction, but Illumina has been unwilling to participate in substantive discussions," the company said in a statement.

The Swiss company also said it will nominate a slate of independent candidates for election to Illumina's board.

Roche said the deal will be financed from available cash and borrowings under its credit facilities and will not require a financing condition.

"It is our strong preference to enter into a negotiated transaction with Illumina," Severin Schwan, Chief Executive of Roche Group, said in a statement.

Illumina was not immediately available for comment outside regular U.S. business hours.