Factors influencing the efficiency of generating genetically engineered pigs by nuclear transfer: multi-factorial analysis of a large data set

Table 2 Stratification and data distribution of the investigated experimental factors

Factor	No. of embryo transfers (%)
Season ¹
- Spring	39	(20.2)
- Summer	59	(30.6)
- Autumn	58	(30.0)
- Winter	37	(19.2)
Type of genetic modification ²
- de novo - AGT	57	(29.5)
- HR	48	(24.9)
- replication of transgenic pig	88	(45.6)
Donor cell source ³
- MSC	36	(18.7)
- PF	24	(12.4)
- FF	51	(26.4)
- KC	82	(42.5)
Cloning rounds
- 1 time	110	(57.0)
- 2 times	62	(32.1)
- 3 times	21	(10.9)
Selection of SCNT embryos for early development ⁴
- no selection	45	(23.3)
- selection after 1 day	13	(6.7)
- selection after 2 days	15	(7.8)
- mixed selection	120	(62.2)

Additional file 3 and Additional file 4 show in more detail the distribution in season and embryo selection of specific SCNT configurations with respect to genetic modification, donor cell source and number of cloning rounds.
¹ Embryo transfer date.
² AGT: additive gene transfer, HR: homologous recombination.
³ Mesenchymal stem cells (MSC), postnatal fibroblasts (PF), fetal fibroblasts (FF), and kidney cells (KC).
⁴ No selection: all SCNT embryos transferred, selection for 1 day: 1-cell stage SCNT embryos transferred, selection for 2 days: 2-cell to 4-cell stage SCNT embryos transferred, mixed selection: mixed SCNT embryos transferred (no selection/1 day and 1 day/2 days).

ISSN: 1472-6750