HAT (hypoxanthine, aminopterin and thymidine) is used for selecting the hybridomas based on the following I. Only hybridoma will grow since it inherited the HGPRT genes from B-cells and can synthesize DNA from hypoxanthine. II. Myeloma cells will not grow in cultures since de novo synthesis is blocked by aminopterin and due to the lack of HGPRT enzyme.

HAT medium selection works through two complementary mechanisms. Statement I is correct: hybridomas inherit HGPRT (hypoxanthine-guanine phosphoribosyltransferase) from B-cells, enabling them to use hypoxanthine for nucleotide synthesis when de novo pathway is blocked. Statement II is correct: myeloma cells are HGPRT-deficient, so when aminopterin blocks de novo synthesis, they cannot synthesize DNA and die. The combination selects for only hybridomas (B-cell + myeloma fusion) that survive because they have immortality from myeloma and HGPRT from B-cells. Therefore, option C (both I and II are true) is correct.