TABLE 1.

Use of chimeric receptors FC-1, FC-2, and FC-4b

Viral phenotypea	No. of isolates	% of replicating isolates using:
		FC-1		FC-2		FC-4b
		p24 onlyb	p24 + syncc	p24 only	p24 + sync	p24 only	p24 + sync
R5 nonswitch	64	20	2	3	63	22	17d
R5 switch	109	37	8	6	79	17	63
CXCR4 using	73	14	51	7	89	0	100

^aR5 nonswitch, R5 isolates from patients with CCR5-using virus throughout the study; R5 switch, R5 isolates preceding the acquisition of CXCR4-using virus in patients with a detected switch to X4, R5X4, or multitropic virus; CXCR4 using, X4, R5X4, or multitropic isolates from the patients in the R5 virus switch group.

^bp24 only, p24 antigen production but no syncytium formation.

^cp24 + sync, syncytium induction and p24 antigen production.

^dSignificantly lower numbers and percentages of replicating R5 isolates were obtained from nonswitch virus patients than from R5 switch virus patients when two isolates from each patient were analyzed as described for Table 2 (P = 0.01, chi-square analysis).