This mix of normal and transformed chloroplasts are defined to be "heteroplasmic" chloroplast population.[5] Stable gene expression of the introduced gene requires a "homoplasmic" population of transformed chloroplasts in the plant cells, where all the chloroplasts in the plant cell has successfully integrated the foreign genetic material.[5] Typically, homoplasmicity can be achieved and identified through multiple rounds of selection on antibiotics.[5] This is where the transformed plant tissue are grown repeatedly on agar plates that contain antibiotics like spectinomycin.[5] Only plant cells that have successfully integrated the gene cassette as shown above will be able to express the antibiotic resistance selectable marker and therefore grow normally on agar plates containing antibiotics.[5] Plant tissue that do not grow normally will have a bleached appearance as the spectinomycin antibiotic inhibits the ribosomes in the plastids of the plant cell, thereby preventing maintenance of the chloroplast[5] However, as heteroplasmic population of chloroplasts may still be able to grow on agar plates effectively, many rounds of antibiotic selection and regrowth are required to cultivate a plant tissue that is homoplasmic and stable.[5] Generation of homoplasmic plant tissue is considered to be a major difficulty in transplastomics and incredibly time consuming.[2]