The 9-cis β -carotene has been shown to be a precursor of 9-cis retinoic acid both in-vitro in human intestinal mucosa and in-vivo in a ferret, perfused with 9-cis b-carotene. The night vision, as measured objectively by electroretinography (ERG) more than doubled in six patients tested following treatment. The visual field was also improved significantly. In a more recent study the investigators treated 29 retinitis pigmentosa patients with the 9-cis b Carotene algae Dunaliella Bardawil in a double masked placebo control cross over trial. Significant improvement in retinal function was recorded in 34% of the patients.