A hospital in Dublin has begun a review of about 3,500 BRCA cancer gene tests after a woman was incorrectly told she did not have the gene.
The misdiagnosis involved an adult patient at Our Lady's Children's Hospital in the Crumlin area.
The woman is now being treated for ovarian cancer and is described as being very ill.
The hospital has apologised and said it was considered to be an "isolated incident caused by human error".
It has undertaken a review of 3,500 tests as a precautionary measure.
'No cause for concern'
A report in The Sunday Times Ireland edition said that the woman had gone for a blood test at Our Lady's Children's Hospital in 2009.
The sample was sent to a British hospital, which gave a positive result for the presence of a BRCA gene, meaning a patient has a much higher risk of developing breast or ovarian cancer.
It informed Our Lady's, but the result was not passed on to the patient.
The woman's solicitor, Caoimhe Haughey, said her client was seriously ill and was seeking an independent review.
In a statement to BBC News NI, the Children's Hospital Group said it "apologises to the woman at the centre of this transcription error and regrets the series of events that led to her current difficult situation".
"All facts in this incident currently point to the fact that a transcription error of a genetic test result occurred.
"This is currently considered to be an isolated incident caused by human error.
"As a precautionary measure, a review by Our Lady's Children's Hospital, Crumlin, is currently underway of all transcriptions of BRACA tests to ensure tests results were transcribed correctly and that no similar transcription error has occurred.
"We want to offer reassurance to other patients who have undergone testing that this is not a testing error and therefore there is no cause for concern or distress," added the statement.
It comes as the Irish health service continues to re-test patients for smear results following the cervical cancer check scandal, which has led to the deaths of 20 patients who received incorrect test results.