(CNN) A freezer malfunctioned at the Children's Hospital Los Angeles, destroying stored blood stem cells for 56 patients, some of whom are fighting cancer.

In a statement Wednesday, the hospital apologized and said it has sent letters to all affected families and set up a phone line for parents to call if they have any questions. It said no child's health is in danger.

"These blood stem cells, all of which were collected from patients, were in long-term storage. We routinely collect cells early in a child's course of oncology therapy for possible future use. When excess cells are not needed, they remain in storage in the unlikely event that they could become helpful in the future," the hospital said.

The freezer's temperature sensors failed and the safeguards put in place were insufficient, the hospital said. It has since implemented several changes, including replacing the malfunctioned freezer, upgrading the sensor monitoring and alert system, and double-checking all power supply sources.

Elizabeth Anderson, whose son was one of those affected, told CNN affiliate KABC that the hospital addressed the letter to her son notifying them of the failure. Her son, Sean, said he went through a grueling process to get the stem cells, and burst out crying when he read the letter.

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