The present study presents cytogenetics/cytology of haploidization in the origin of a new, fast growing diploid, small cell-type (F-dPCs). The sequence of events was haploid groupings of the chromosomes in normal, human metaphase cells, followed by genomic doubling to homozygousdiploidy. These events were responses to DNA replication stress fromamino acid glutamine deprivation. Importantly, these homozygous cells outgrew normal fibroblasts in 2 - 3 passages—they had gained proliferative advantage (GPA), presumably from loss (LOH) of tumor suppressor genes. They were morphologically changed cells with rounded nuclei that grew in a “streaming” growth pattern and with changed form and size of mitosis, similar to some hyperplasias. The grouping of the chromosomes in metaphase cells was asymmetric with a narrow range around the median (23) (no micro-nuclei), suggesting genetic control. The root-origin of haploidization was evidenced by maternal and paternal genomes occupying separate territories in metaphase cells, which assumedly permitted independent segregations of bichromatid chromosomes. In near-haploid ALL-L1 leukemia the loss of virtually, whole chromosomal complements was judged by SNP array analyses, as a primary event before genomic doubling to hyperdiploidy with LOH. From the present data such specific, non-random loss of chromosomes strongly suggested, a haploidization process capable of genomic doubling, as observed for the “birth” of the small, F-dPCs. This suggestion was supported by this type of leukemia being the L1-type, where L1 signifies small cells. The possibility now exists that a tumorigenic process can be initiated directly from diploid cells through haploid (near-haploid) distributed chromosomes in normal metaphase cells. This event followed by monosomic doublings to UPDs would lead to massive LOH and a return to para-diploidy, a frequent occurrence in many types of tumors. The present simple, cultural derivations of the extraordinary F-dPCs allow GPA-identification and experimental manipulations, perhaps relevant in a vaccine program.