Rare mutations of the PIK3CA gene in malignancies of the hematopoietic system as well as endometrium, ovary, prostate and osteosarcomas, and discovery of a PIK3CA pseudogene. Lipid kinase PIK3CA mutations have been described in several cancers. They clustered in two 'hot spots' located in helical (exon 9) and kinase (exon 20) domains associated with increased kinase activity strongly suggesting oncogenic potential. Mutational analysis of previously unexamined tumors showed an amino acid change from threonine to alanine (T1025A) in exon 20 in one of 28 endometrial cancer samples and 6 endometrial cell lines. Additionally, a silent polymorphism (T1025T) was found in two of 20 MDS samples, one of 43 NHL samples, two of 40 osteosarcoma samples and Ishikawa. The polymorphism was established by identifying two of 92 normal samples with the same change. No PIK3CA mutations were found in acute myeloid leukemia (AML), myelodysplastic syndromes (MDS) and non-Hodgkin lymphomas (NHL) as well as in osteosarcomas, prostate and ovarian cancer samples. Additionally, a previously unidentified PIK3CA pseudogene spanning exons 9-13 on chromosome 22 was discovered.