How do Klein and Maxson describe the nature of the gang crime nexus?

Klein and Maxson use a variety of terms to describe the gang crime nexus in their book, but the unifying theme is that gangs constitute the central point of the majority of criminal activity in a given area. Obviously, there will be plenty of auxiliary crime, but looking at a statistical summary, gang members will constitute a surprising majority of criminal activity.
In their work, Klein and Maxson show that gang members are responsible for a large number of multiple incarcerations, meaning that criminals are not simply one-time offenders. Gang members tend to commit multiple crimes and resist rehabilitation. Additionally, this typically implies that those people have committed multiple other crimes for which they were not convicted.
Statistically, the two also show that gang members are responsible for all sorts of violent crime, from drug trafficking to rape to murder and many other types of criminal activity. The natural conclusion is that gangs constitute a sort of "nexus," the unifying point of major criminal activity.

In April of 1996, authors Cheryl Maxson and Malcolm Klein published a National Institute of Justice (NIJ) research report titled "Gang Structures, Crime Patterns, and Police Responses." The report was based on a study that looked at a series of interconnections linking the most routine patterns of street gang structures with patterns of crime.
The authors sourced testimony from gang experts in nearly sixty cities, gang-related crime data from 110 cities, and additional data culled from several older national surveys. The study essentially resulted in five major findings, which can be summarized as follows.
"Traditional gangs" that were the greatest subject of past crime studies were not as common as researchers had originally thought. Variations in gang type did not correlate with any traits or aspects of their various cities. Gangs that dealt drugs were actually a relatively small percentage of all gangs.
Ethnic differences among gangs were not in line with previous researchers' estimates from the 1990s. Official arrest records did not necessarily paint an accurate portrait of the relation between crime patterns and gangs.
However, the authors did admit that it was possible that the resulting gang typology captured in the study was limited to a brief moment in the ever-shifting nature of street gangs.

In chapter 2 of Street Gang Patterns (Oxford University Press, 2010), Klein and Maxson report statistics on gang arrest profiles in 1994. Their evidence suggests that the relationship between gang membership and crime is "robust across different measures of offending" (page 73). The authors used both self-report measures and crime statistics and found that gang members are more likely to be arrested and to be arrested several times. Therefore, the nexus between gangs and crimes is strong.
The authors also cite research by Thornberry (1998), who looked at longitudinal samples of delinquency in Rochester, Seattle, and Denver. This research found that gang members commit crimes, particularly serious crimes, to a disproportionate degree in every category in every city studied. For example, gang members committed 85% of all robberies carried out by youth in Seattle, 79% of all serious violent crimes, and 87% of all drug sales in Denver (page 73). In Rochester, gang members carried out 86% of all serious violent crimes and 70% of drug sales. In addition, the authors cite research that suggests that gang members are also more likely to engage in non-violent crimes (page 73).
The authors describe the nexus between gangs and crimes as "a cafeteria of delinquency" (page 74), meaning that gang members' offenses fall into every category (similar to the way in which cafeterias serve all varieties of food). This pattern also holds true for gang members of different genders and from different races/ethnicities. While girls in gangs have lower crime rates than boys in gangs, female gang members have higher rates of delinquency than non-gang males, according to many studies. Therefore, the nexus between gangs and crimes is strong across genders and races/ethnicities.