Because California Code of Regulations, title 15, section 3378 (identification of inmates as prison-gang affiliates) is a quasi-legislative rule, courts will defer to the interpretation of the regulation by the California Department of Corrections and Rehabilitation (CDCR). Petitioner, serving a life prison term, was validated by CDCR as an associate of the Mexican Mafia prison gang under section 3378. The classification was based on the discovery of photocopies of several drawings distinctive to the gang that were found in his cell, some of which were signed by validated affiliates of the gang. Petitioner filed a habeas petition in the Court of Appeal challenging his validation and the court granted relief. The court’s opinion rested on a disagreement with the CDCR over the interpretation of the CDCR’s own regulation addressing the validation of gang associates. Reversed and remanded. The Legislature has given CDCR broad authority for the discipline and classification of prison inmates, including promulgating regulations governing these subjects. Section 3378 is a quasi-legislative rule promulgated by the CDCR to identify and manage inmates who are affiliated with prison gangs. Courts will defer to an agency’s interpretation of its own regulations, particularly when the interpretation implicates areas of the agency’s expertise. Here, the Court of Appeal interpreted section 3378, subdivision (c)(4) to require proof that an inmate formed a reciprocal or mutual relationship with a validated gang affiliate in order to establish a direct link, via the source item category of association, with that gang affiliate. The CDCR construed the regulation more broadly and did not require evidence of a reciprocal interaction. The court erred by not deferring to the CDCR’s interpretation or acknowledging CDCR’s expertise in prison management. The CDCR’s policy of relying on unilateral inmate contact to satisfy the direct link to a validated gang affiliate is not clearly unreasonable.