Researchers at the Institute of Physical and Chemical Research (Riken) recently announced they have found a much larger difference between human and chimpanzee genes than the conventionally accepted level.

They say the difference, based on a yet-to-be-completed genome study of the primate most closely related to humans, is about 15 percent.

Yoshiyuki Sakaki, director of the Riken genome project, said, "We believe there might be similar degrees of difference in other chromosomes."

Sakaki said the Riken researchers found the difference by mapping the complete base sequence of the chimpanzee chromosome 22.

The research team -- consisting of scientists from Japan, South Korea, China, Taiwan and Germany -- is based at the Human Genome Research Group of Riken Genomic Sciences Center.

Sakaki said the chimpanzee chromosome 22 was mapped at a precision rate of 99.998 percent. The chromosome is considered equivalent to human chromosome 21, which has the most advanced research record among human chromosomes.

The comparison of the two sets showed that the basic sequence of the chimpanzee chromosome was 1.69 percent different from that of the human equivalent. An earlier Riken study put the difference of the complete basic sequence at 1.23 percent.

When the researchers compared 235 of about 300 genes in the two sets of chromosomes, Sakaki said, they found differences in 36 genes, their differences marked by variances in the genes' structures.

The difference is believed to affect the production of protein, in ways such as making the chain of amino acids shorter or creating room for more.