A recessive mutant with white leaves was identified in a screen of a population of T-DNA-taggedArabidopsis thaliana plants. The mutation is lethal, but plants develop almost to maturity under sterile conditions. The white areas in leaves are devoid of developed chloroplasts, but the plants frequently develop green sectors which contain green chloroplasts. Molecular characterisation of the affected gene revealed that the mutant is allelic topale cress (pac), a recently described mutation, and was therefore namedpac-2. Sequencing of cDNAs and the genomic region revealed several noteworthy features of this genetic locus. Inpac-2 the T-DNA had inserted in the region of the promoter and abolished transcription of thePAC gene completely. Cytokinin induced greening in mature, white homozygouspac-2 plants, and therefore is likely to be responsible for the greening observed in callus and shoots induced on roots from such plants. However, thePAC transcript was found to be absent in both white leaves and green callus. Thus, since cytokinin induced greening in the absence ofPAC RNA this plant hormone appears to be able to bypass PAC function.