The probability of having CD4+ < 350 cells/mm3 also seemed to be associated with being an injection drug user (hazard ratio = 0.03), a variable positively correlated with HCV co-infection (correlation index = 0.82), whereas the gender, age and time since HIV infection was detected did not seem to be determinant of CD4+ falling below 350 cells/mm3. This evidence concerns the gene CD4 and coinfection.