The number 13 tattooed on the nape of the neck may hold personal significance that is not immediately apparent, such as a birthdate, so one should not jump to conclusions. However, it is possible that the tattoo is a sign of affiliation with the MS-13 street gang.
MS-13 gang members are known to sometimes display the numerals "13" or the Roman numeral variant "XIII" on various parts of their bodies. A tattoo of this type should not be automatically considered a sign of gang involvement, however. Aside from the possibility of 13 representing a date of personal significance, it is also widely regarded as an unlucky number and is embraced by some subcultures.