Skylark Bio is conducting a study of an investigational new drug called SKY-GJB2 and its delivery device, the SKY-CAT. SKY-GJB2 is a gene therapy that is being developed to treat children who have hearing loss due to changes in the GJB2 gene (also referred to as Connexin 26). The SKY-CAT is a device that delivers the gene therapy.

The purpose of this study is to:

Learn about the safety and tolerability (does not cause ongoing discomfort) of SKY-GJB2 Evaluate the efficacy of SKY-GJB2 (how well is works)

Inclusion Criteria:

• Male or female aged 9 months to 7 years at the time of gene therapy administration
• Subjects must have at least two (biallelic) pathogenic or likely pathogenic variants in the GJB2 gene
• Bilateral sensorineural hearing loss as assessed by ABR. Hearing loss at ≥85 dB HL for at least one of the frequencies (500-4000 Hz) in the study treatment ear.
• Subject's parent(s)/guardian(s) provide informed consent before the initiation of study-related procedures.
• Subject is able and willing to comply (or provide assent if old enough) will all study requirements.

Exclusion Criteria:

• Subject has non-GJB2 mediated hearing loss including genetic, syndromic, or non-syndromic hearing loss that is not associated with GJB2 mutations.
• Subject has autosomal dominant nonsyndromic hearing loss due to GJB2 mutation.
• No response on ABR testing.
• Bilateral Cochlear Implants.