However, a study conducted on the tobacco plant in 2007 has disproved this theory. Led by Ralph Bock from the Max Planck Institute of Molecular Plant Physiology in Germany, researchers studied genetically modified tobacco in which the transgene was integrated in chloroplasts. A transplastomic tobacco plant generated through chloroplast mediated transformation was bred with plants that were male sterile with an untouched chloroplast. The transplastomic plants were engineered to have resistance to the antibiotic spectinomycin and engineered to produce a green fluorescent protein molecule (GFP). Therefore, it was hypothesized that any offspring produced by from these two lines of tobacco plant should not be able to grow on spectinomycin or be fluorescent, as the genetic material in the chloroplast should not be able to transfer via pollen. However, it was found that some of the seeds were resistant to the antibiotic and could germinate on spectinomycin agar plates. Calculations showed that 1 out of every million pollen grains contained plastid genetic material, which would be significant in an agricultural farm setting. Because tobacco has a strong tendency towards self-fertilisation, the reliability of transplastomic plants is assumed to be even higher under field conditions. Therefore, the researchers believe that only one in 100,000,000 GM tobacco plants actually would transmit the transgene via pollen. Such values are more than satisfactory to ensure coexistence. However, for GM crops used in the production of pharmaceuticals, or in other cases in which absolutely no outcrossing is permitted, the researchers recommend the combination of chloroplast transformation with other biological containment methods, such as cytoplasmic male sterility or transgene mitigation strategies. This study showed that whilst transplastomic plants do not have absolute gene containment, the level of containment is extremely high and would allow for coexistence of conventional and genetically modified agricultural crops.