Table 2.

Mice with MMTV can be identified by scent in the absence of tumors

Group	Odor Source†	Training trials‡		Generalization trials§
		C	NC	C	NC
1	Female mice	316	96**	40	18**
2	Females' urine	191	58**	24	11*
3	Male mice	201	59**	28	13*
4	Males' urine	315	84**	44	20**
5	Rederived female mice	205	58**	29	13*
6	Rederived females' urine	402	54**	58	18**
7	Genetically transmitted virus; male urine	370	58**	54	26**

Training and testing procedures are described in text and refs. 11 and 12. 

^† Odor source mice are described in Table 1. Mice were used either as odor sources themselves (groups 1, 3, and 5) or to provide urine (groups 2, 4, 6, and 7). For mice as odor donors, one mouse was housed in a wire mesh cage that fit comfortably within the odor box. For each trial, different pairs of individual mice, differing in MMTV status, served as odor generators. Urine odor stimuli were obtained from mice by gentle abdominal pressure (males) or in metabolic cages (females). Usually, a single mouse provided enough urine (0.2–0.3 ml) to cover the bottom of a 3.5-cm-diameter Petri dish, but sometimes two mice were needed. Fresh samples from different donors were used for each run. Generally, on each day of testing, a given combination of two numbered donors was not repeated. Virtually all MMTV⁺ female odor donors, and no MMTV⁻ female odor donors, developed mammary tumors after being bred following completion of these trials. 

^‡ Trained (sensor) mice were either C57BL/6Boy or congenic B6-H-2^k adult females; four sensors were assigned to the distinctions of groups 1, 2, and 3; three sensors to group 4; and five sensors to groups 5, 6, and 7. Approximately half the trained sensors were reinforced in the Y-maze for MMTV⁺ odor, the remainder for MMTV⁻ odor. C, Concordant with training (“correct”); NC, Non-concordant (“not correct”). 

^§ Generalization trials were conducted with odors of MMTV⁺ and MMTV⁻ mice or urines not previously encountered by the sensors. Sensors were never reinforced for concordant responses in these trials, allowing for blind testing, and thus providing a critical test of discrimination between presence and absence of MMTV. 

^*, P < 0.05 (binomial test); 

^**, P < 0.01; 

^**, P < 0.001.