Swiss drug maker Roche today increased its hostile cash offer for Illumina by 15 per cent, valuing the US-based gene sequencing company at about $6.7 billion.

''Based on our discussions with Illumina shareholders we have seen interest to accelerate the takeover process,'' said Severin Schwan, CEO of Basel, Switzerland-based Roche.

The sweetened offer came after Roche held ''a number of productive discussions'' with Illumina shareholders in recent weeks, said Roche.

Roche has now increased its offer from $44.50 a share or $5.7-billion to $51 a share, and Illumina investors have until 20 April to tender their shares to the revised offer.

The board of San Diego, California-based llumina had rejected Roche's initial offer made in January, forcing the Swiss pharmaceuticals giant to take its offer directly to shareholders.

"Roche's preference continues to be a negotiated transaction. We look forward to the possibility of a swift completion that offers immediate value to Illumina's shareholders," said Schwan.

The revised offer represents a 15 per cent premium to Roche's offer on 25 January and a premium of 88 per cent over Illumina's closing stock price on 21 December 2011.

Roche said that it is confident that the improved offer would turn round Illumina's board, which has been consistently resisting the takeover, citing that the initial offer as being ''grossly inadequate.''

Illumina had earlier rejected Roche's unsolicited hostile offer as it failed to properly value its existing and coming products.

''Roche - an Illumina competitor with a record of pursuing hostile acquisitions – is trying to buy Illumina at a lowball price and capture our future growth and value potential for its own stockholders,'' Illumina had said in a statement.

Illumina also bolstered its takeover defences by adopting a poison-pill defence, which gives investors the right to acquire shares and make the hostile takeover prohibitively expensive.

The rejection has triggered a proxy battle ahead of Illumina's shareholders' meeting scheduled to be held on 18 April.

But Roche has in the past successfully played a waiting game while making major acquisitions. Roche waited patiently for seven months to acquire Ventana Medical Systems for $3.4 billion in 2008, after legal battles to counter a poison-pill defence (See: Roche to acquire Ventana for $89.50 per share).

Illumina today advised its shareholders to ''defer taking any action,'' and said that it would review the revised offer and make a recommendation in due course.

Founded in 1998, Illumina began offering SNP genotyping services in 2001 and launched its first system, the Illumina BeadLab, in 2002, using GoldenGate Genotyping technology.

Illumina manufactures instruments and products used to study DNA, which allow researchers to study an individual's genetic makeup, sequence genomes, and determine the specific genetic causes of diseases.

Illumina holds a 60-per cent share in the next-generation sequencing market, a rapidly growing segment in the life science tools industry. The company believes that around 90 per cent of the world's sequencing output is produced on Illumina instruments.

Illumina has the widest commercial product portfolio in the life science tools industry, with over 2,300 peer-reviewed sequencing-related publications and more than 8,000 peer-reviewed publications using its technology.

The potential acquisition would make Roche a market leader in gene sequencing, a procedure that helps identify which patients benefit from a given drug.

Roche, the world's biggest maker of cancer drugs, wants to shift Illumina's gene-sequencing technology from academic research labs into routine medical use. The acquisition would build its portfolio of health diagnostics products and most importantly, allow it to target its medicines toward individual patients.