Urbina-Mejia v. Holder, http://www.ca6.uscourts.gov/opinions.pdf/10a0063p-06.pdf
In this published decision, the Sixth Circuit found that former El Salvadoran gang members can constitute a particular social group. It is a characteristic that is impossible to change short of rejoining the gang. Note, that the noncitizen did not have any gang tatoos.
In this REAL ID Act case, the IJ found that the claim, while credible, lacked corroborating evidence. The court found that some of the missing evidence was unlikely to be probative, such as letters from the noncitizen's much younger siblings or the family members who were in the U.S. when these events occurred. However, a note from his father, who remains in El Salvador was available but not provided "because no one asked him for it."
The noncitizen presented an expert witness, whose testimony was somewhat discounted by the IJ as "mere opinion." The court found that his testimony was corroborative evidence. Still, under the REAL ID Act standard, the "immigration judge’s conclusion that the available evidence was insufficient to compel a finding that Urbina-Mejia would more likely than not suffer persecution upon removal was reasonably based on substantial evidence."
In what might be a fatal blow to similar claims, the noncitizen's admission that he robbed and extorted money and sometimes assaulted victims, and was taught to use a gun, meant that he was ineligible for withholding of removal because he committed a serious nonpolitical crime outside the U.S. It did not help that he was under the age of 18 when this happened or that there was some amount of coercion (the IJ noted that he shared in the proceeds of his crimes). The concurrence would have denied on this basis alone without reaching the particular social group or corroborating evidence issues.